According to many sources, including Reuters, Israel is not a member of the Non-Proliferation Treaty (NPT). Why hasn't Israel been pressured and sanctioned in a similar manner to Iran when it actually does have nukes - but all of the pressure is on Iran that according to IAEA and other sources doesn't even have nuclear WMD.
In theory, membership of the NPT is voluntary. Members gain the benefit of civilian nuclear technology exchanges up to a point just short of nuclear weapons capability in exchange for openness and the pledge not to cross that point. (People who pressure Iran want more than just ordinary NPT membership from them ...)
In practice, failing to do what other countries want leads to consequences. The severity of those consequences depends on the strength of the involved countries.
- There are countries putting pressure on Israel. They are not strong enough to make Israel give in.
- There are countries putting pressure on Iran. They are not strong enough to make Iran give in.
There is a tendency in Washington and in parts of the US press to equate "America," "the West," and "the international community." They're not the same. The US and their allies are just one very powerful block -- countries put to the question of either not trading with Iran or not trading with the US tend to value trade with the US more, and stop trading with Iran.
That's not primarily based on the NPT and similar principles. It is based on the fact that the US can decide not to trade with countries it dislikes, and decide whom it dislikes. No different, really, than the two blocks during the Cold War, except that the relative strengths are less balanced.
As the answer by @o.m. correctly points out, membership in the NPT is voluntary, and forcing any country to join it would constitute a breach of its sovereignty. Of course, de facto weaker countries are forced to do things by more powerful ones all the time, but this is usually couched in more diplomatic language.
Note also that Israel is not the only country outside the NPT - India and Pakistan are in a similar situation, while both possess nuclear weapons. It seems that NPT does not have a provision for bringing in a country possessing nuclear weapons - its nuclear members are restricted to the five countries that possessed the weapons in 1968, while everyone else is allowed to join as a non-nuclear member. Asking a nuclear-armed country to disarm is a non-starter.
Therefor, the usual practice in such cases is to introduce some additional agreements, engage the countries in question in peaceful nuclear cooperation (like India-United States Civil nuclear agreement), and demand public commitments to safe behavior. Specifically where Israel is concerned:
- Israel is a member of International Atomic Energy Agency
- Israel adheres to the policy of nuclear ambiguity - it has never confirmed to possess nuclear weapons, although it is widely believed that it has them.
- The standard formula used by Israeli politicians is that
Israel will not be the first country to introduce nuclear weapons to the Middle East.- which might mean that Israel either doesn't have such weapons or will not use them first.
- Israel supports establishment of a Middle East Zone free of weapons of mass destruction. (see Israel and weapons of mass destruction)
The OP says:
Why hasn't Israel been pressured and sanctioned in a similar manner to Iran when it actually does have nukes - but all of the pressure is on Iran that according to IAEA and other sources doesn't even have nuclear WMD.
It is not quite clear what the OP refers to here - Iran has been a member of the NPT since 1970 (from before the Islamic Revolution), and used this membership to obtain help in developing peaceful nuclear energy (notably from Russia). In other words, since 1970 Iran has committed itself to using nuclear energy only for peaceful purposes, and this policy was upheld after the Iranian revolution.
Iran is currently in violation of the NPT, due to breaking the NPT safety benchmarks (like the level of Uranium enrichment and development of nuclear capable rockets), as well as because of its refusal to allow verification of its compliance with the NPT rules (refusing access to IAEA inspectors) - hence the pressure. On the other hand, Israel had never joined and therefore never violated the NPT.