Because... er, because... lol

The reality is that any region may legitimately seek independence from the country it's part of. Ideally, if the separation goes ahead, it does so on peaceful terms. But it could also involve armed conflict. Armed conflict in pursuit of independence can be a legitimate and lawful way to achieve the goal.

What the West (and pretty much no one else) resents about Crimea is that the independence of a breakaway region is supposed to happen 'organically,' and over a long period of time. That wasn't the case in Crimea, as Russia quickly facilitated the secession of Crimea from Ukraine. However, the proof of the legitimacy of that secession is the peaceful manner in which it unfolded, and the peace that has existed there since.