The US government action was deemed illegal under the terms of a treaty (in that case the Treaty of Amity of 1955), which the US freely negotiated and entered into and ratified. What is the point of any nation entering into treaties with the US, if it's going to just ignore and violate them at the whim of whoever happens to be president at the time?

As a condition of easing sanctions, Iran filled the rector core of it's active nuclear reactor with concrete and was found to be in compliance with all the terms of the agreement. Good luck getting any other nation to enter into negotiations like that with the US in good faith again.

The Iranian regime is awful. They're appalling extremists and oppressive tyrants. No argument there, but that's on them. It doesn't justify recklessly violating treaties and unilaterally breaking agreements, and establishing a track record of doing so. If anything, demonstrating a track record of honesty and respect for straight dealing is even more important when dealing with such people.

So are you saying it's not illegal, or are you saying there won't be any consequences for this crime?

This is of course true, as law enforcement depends on what is called "the monopoly of violence". And this is of course impossible when dealing with major powers in the current world.

Doesn't mean it's legal. Plus the UN is not toothless, it has been very effective at achieving what is was founded for, preventing another world war.