Exigent circumstances.
If the authority of the state is illegitimate, its laws and decisions are not binding on its citizens.
And in fact Kiev faces a crisis of legitimacy and this was the reason behind the Crimean decision to secede from Ukraine.
What “crisis of legitimacy”? The Rada is there, there is an interim president.
But that is beside the point. Governments may change, but constitutions are above that.
It is very true, — as the posters here never tire to comment, — that what some commission decides is not the facts on the ground. However, to those who would like to know not only the military facts but also legal facts: here they are. The referendum is in plain violation of any reading of the Ukrainian constitution, of the Crimean provincial constitution, and the laws of the EU.