This is actually on purpose to additionally stage realism - the medikit cannot be used on the soldier himself because of hesitation. In real life, people hesitate before they poke themselves with a sharp and painful needle.
In the game, this hesitation is so long that it would take too many TU's to ever overcome this hesitation factor.
But even so, the Medikit is a little bit buggy, because it seems to have unlimited ammo. Can you also heal aliens with a medikit??? :?