I'll give you the Eurozone... Makes it impossible to devaluate and much harder to put capital controls in place (though Cyprius manage to do so while being in the Eurozone).

However, what Iceland did was still unilateral. They didn't ask for anybody's permission and didn't expect their decision to be accepted.

Laws and treaties still need to be enforced and those countries are still sovereign. What happens if a country does something that is against EU law ? At worst, they may be expelled from the EU.

So it's really a matter of comparing the advantages of EU membership to the cost of the path of action that was taken.

It's still a political decision.