Well, let's see:
- both elections and referendum are supposed to be used to either gauge or maintain legitimacy of respective processes (elected representatives or enforcement of whatever it is decided by referendum)
- both are ideally supposed to be best performed without outside interference (which existed in both cases), and with equal access for parties representing different viewpoints, including "restoring/maintaining status quo" (which wasn't really provided in both cases)
- in either case forces of previous government were suppressed by force (threat of force in case of Crimea)
There are many cases where US accepted things that weren't accepted by UN at large (like Honduran coup of 2009 -
currently used against Clinton - quote "Though all international organizations called the Honduran coup illegitimate, and refused to recognize the leader chosen by its junta, the Obama Administration, after more than a month of indecision on this matter, finally came out for Honduras’s fascists." (those junta members/powerful families of Honduras made contributions to Clinton fund before coup, which is seen as reason for her support of them) ).
Unilateral actions like this are not really uncommon. Not every action needs world approval.
Both situations were imperfect and "dirty". One outcome was accepted, one wasn't yet de jure (but is pretty much accepted de facto).