But it would force Russia to deal with the actual populace instead of the government, if the region is as heavily pro-Russian as Russia claims the truth will come out real fast. Would it then be a Russian crisis or a Ukrainian crisis?
However, it would lend an awful lot of legitimacy to Russia's claims of just being their to protect the people if Kiev did such a thing, while not endearing anyone in the peninsula to the Kiev based government. Kiev would prefer to have the world view this purely as an issue of sovereignty; introducing questions about the ethnic makeup of the area poses a decent argument for self determination, which Kiev would like to avoid. Of course, none of this condones Russia's actions, but someone needs to tell RT that, as they continue to spout their propaganda.
You seem certain that the truth would favor Ukraine ;) If we hypothetically assume the population ends up celebrating their liberation, how is Ukraine ever going to have any credibility demanding the region back?