>

It's posturing. A calculated move to make the enemy work overtime to figure out their contingencies as well as their diplomatic response. 
Obviously, from an ethical point of view, Ukraine ought to be free to launch missile strikes on Russian soil.  I think it's something that the west is saving for the possibility of a near-russian victory.
Interesting. Though of course, not ruling something out, and actually doing something are two very different things. Still, the fact that they're putting the option on the table is telling.