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.“
After the Russia has invaded Ukraine and occupied Crimean peninsula, it became clear that weak reaction to the 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 yours interests is The Bomb.
Having that said, the signatories of the Budapest Memorandum do help Ukraine in unprecedented amounts:
- U.S. has introduced devastating sanctions against economy of the Russia, which led them to losses of $0.2 trillion (official information by the russian propaganda) to $0.75 trillion (analysts) only during Q2-Q3-Q4 2014;
It should be mentioned, the U.S. and European economies also suffer from broken contracts and missed opportunities, so this step is not as easy as it seems to be;
- France has cancelled a €1.2 billion program of selling to the Russians an assault weapon, two helicopter carriers "Mistral";
- The Russia has been expelled from G7 and G20 — by itself, it's a huge political defeat;
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)
U.S., Canada, and European countries (on individual basis or within NATO) are providing Ukraine with non-lethal military support;
- (update 2015-05-16) U.S. Senate Armed Services Committee and The House passes defense policy bill that includes $300 million in military assistance to Ukraine, incl. lethal weapons;
- U.S. shares their intelligence data, including satellite reconnaissance, with Ukraine's General Staff (an unprecedented action);
- UN is currently discussing sending UN peacekeeping mission to occupied territories of Ukraine;
- U.S. and NATO instructors are coming to teach Ukrainian army;
- 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 lowered the sovereign rating of the Russia 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;
- update 2016-02-20:
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
Although many see the Russian invasion as just a small Russian-Ukrainian war, it is not. In fact, the Russia has declared war to entire world. This video (Youtube) has been broadcasted on Russian propaganda TV channel on February 8, 2015 during prime time (English subtitles).
Instead of direct ignition of a military standoff on soils of Ukraine, the signatories of Budapest Memorandum have chosen a more effective way of elimination the Russian treat. This includes several aspects:
- Political — expulsion the Russia from international organizations; reducing their role in UNSC;
- Economical — sanctions, oil prices, etc.;
- Criminal — currently open cases:
- A nuclear terrorist act in the center of London: Litvinenko's murder with radioactive Polonium-210;
- Shooting MH17 airplane;
- Shelling Ukrainian cities (including shelling from the Russian territory);
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.