The US had lost its boy Calles in Mexico, it couldnt do a coup. You are acting like the US not doing the worst possible thing means they didn't oppose it. Having written about this specific matter pretty recently, yeah the US hated Cardenas, but didn't invade or anything because Roosevelt was isolationist and it would be the biggest possible violation of the Good Neighbor policy. We are singlemindedly against nationalization, certainly in Mexico, obviously in Mexico. We just didn't have the means to stop it in that case, though we did try.
The US was blindsided by Cardenas, that was the biggest factor there.