Easy: US need Israel's independance as it's their ally and fortpost between them and "crazy" East; US don't need Ukraine's independance and they are afraid of russia's lose as they don't understand how to (or don't want to) deal with defeated russian lands that would become ex-rus but independant once today-russia is beaten.
My latest on the topic:
(Pay attention to the comment from a Brit seeing nothing wrong in russia invading the whole Europe. Sounds nuts for me, but such an opinion has a place to be; I suppose that quite many people, including west gov officials, think smth like this)
