The most accurate answer would be: they do, despite they are not obliged to.
The text of Budapest Memorandum says:
- The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon State party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used;
Many analysts say that neither U.S. nor U.K. officials believed that their lawmakers would ratify an obligating international treaty, so they mutually agreed to make the Budapest Memorandum a political agreement.
Quoting Steven Pifer's interview to CNN:
„…neither the George H. W. Bush administration nor the Clinton administration was prepared to extend a military commitment to Ukraine — and both felt that, even if they wanted to, the Senate would not produce the needed two-thirds vote for consent to ratification of such a treaty.
The Budapest Memorandum thus was negotiated as a political agreement. It refers to assurances, not defined, but less than a military guarantee. U.S. negotiators — myself among them — discussed this point in detail with Ukrainian counterparts so that there would be no misunderstanding.“
At the time, this form of agreement seemed adequate for the political situation. However, after Russia invaded Ukraine and occupied the Crimean peninsula, it became clear that a weak reaction to Russian aggression would ruin the whole system of international agreements of such kind.
As I mentioned in another answer, there are many countries who have or had their nuclear programs: consider Iran, Libya, Pakistan, Egypt, Argentina, Brazil, and South Korea.
They all have territorial disputes with their neighbors, and in some cases these neighbors are already nuclear countries (see, for instance, India versus China).
Obviously, the nuclear states put huge efforts to non-proliferation programs in these countries. The only way how this can be accomplished is by providing these countries with some guarantees, similar to those stated in Budapest Memorandum.
If the signatories of Budapest Memorandum did not help Ukraine, this would be clear indication that
the only way to protect your interests is The Bomb.
Having that said, the signatories of the Budapest Memorandum do help Ukraine in unprecedented amounts:
- Sanctions: The U.S. has introduced massive sanctions against Russia's economy, which led to losses of $0.2 trillion (official information by Russian propaganda) to $0.75 trillion (analysts) only during Q2-Q3-Q4 2014.
It should be mentioned that American and European economies also suffer from cancelled contracts and missed opportunities, so this step is not as easy as it seems to be.
- No More Collaboration: France has cancelled a €1.2 billion arms sale: two "Mistral" helicopter carriers.
- Political Support: Russia has been expelled from G7 and G20 — a huge political defeat.
- Military Support: U.S., Canada, and European countries (on individual basis or within NATO) are providing Ukraine with non-lethal military support.
- Arms: U.S. Senate Armed Services Committee and The House passes defense policy bill that includes $300 million in military assistance to Ukraine, including lethal weapons.
The United States, as Ukraine continues to defend itself against the Russian aggressor, is committed to being a steadfast partner. We have provided $266 million dollars in security and defense-related assistance since Russia’s invasion of Crimea in 2014.
This year , we expect to provide an additional $335 million in training and equipment to our Ukrainian partners. This new assistance, provided by the U.S. Congress, will allow us to expand U.S. training at Yavoriv and Khmelnytskyi, and to deliver needed military equipment to improve Ukraine’s defensive capabilities on the front lines.
— Geoffrey Pyatt, U.S. Ambassador to Ukraine
- Intelligence: U.S. shares their intelligence data, including satellite reconnaissance, with Ukraine's General Staff (an unprecedented action).
- Peacekeeping Mission: UN is currently discussing sending UN peacekeeping mission to occupied territories of Ukraine.
- U.S. and NATO instructors are coming to teach Ukrainian army.
- Financial Aid: IMF and G7 are preparing a new $40 billion package of economic aid to Ukraine.
updated on 2015-03-11: the initial $17bn have been approved.
- Moody's and S&P have already downgraded Russia's credit rating below junk level. By itself, this means no future investments, plus the vast majority of existing investors would demand preliminary closure of investment programs because most credit agreements have covenants linked to ratings assigned by one or more rating agencies.
- Other Aid: U.S. has started many programs to help Ukraine, including:
- Humanitarian assistance.
- Security sector improvement.
- Economic stabilization.
- Energy security.
See this fact sheet for more actions in place. (note, it is a bit backdated, September 2014)*
Although many see the Russian invasion as just a small Russian-Ukrainian war, it is not.
In fact, Russia has effectively declared war on the rest of the world. This video (Youtube) has been broadcasted on Russian propaganda TV channel on February 8, 2015 during prime time (English subtitles).
Instead of the direct ignition of a military standoff on Ukrainian soil, the signatories of Budapest Memorandum have chosen a more effective way of eliminating the Russian treat. This includes several aspects:
- Political — Russia's expulsion from international organizations, reducing their role in the United Nations Security Council.
- Economical — sanctions, oil prices, etc.
- Legal — currently open investigations of acts of war and terrorist activity.
- Humanitarian — helping Ukraine to avoid a crisis caused by foreign invasion.
This looks quite similar to how the world has eliminated the previous Russian project, a militarist "USSR", and the outcome is supposed be similar as well.