Bait and switch. Do you have a problem with that or are you ok when corporations or even nations do it? They built these platforms under the impression from the Venezuelan government that they would be able to own and operate them. After all is said and done Venezuela decides to say fuck you and take it. That is very much a different thing. This has nothing to do with the UN, but everything to do with Venezuela "attacking" our interests.
Why I'd say retaking the platforms is probably in a legal right, what was the original context of the contracts?
Many South American countries were basically forced into extremely unfair trade agreements (>century long whole sale on their most valuable resources) due to the weak position of these countries on the global power game. Their economies couldn't exist without foreign trade with the major powers, and their militaries could not deal with a blockade or invasion. I'd say you probably should withhold a moral standpoint without knowing if the original deal was something like "You will give us the oil at cutright prices for 100 years. If you don't, we will blockade you until you agree."
Venezuela is going to have major reprecussions anyway. Good luck getting many foreign businesses to deal with them or invest in them now. South American countries have historically been very screwed, but they've also historically made very bad decisions with regards to developing a long-term, stable future. Look how far the Asian tigers came in 50 years, while most of the South American countries are moving more in the direction of Africa than the 1st world.