My first response was just to keep snap firing consistent. Giving it some more thought, I have the following observations:
I believe that adopting a kneeling position costs MPs but rising from a kneeling position does not. Assuming that a 0 MP action can be performed while "remaining stationary", you could stand up and fire over the low obstacle and receive the +1 modifier. This isn't "snap firing". Similarly, you could declare yourself walking, stand up, fire, and then use your MPs to kneel down again without snapfiring. In that case you do not get the -1 penalty on firing nor the defensive bonus. That's analagous to walking, spending 2MP to sidestep from behind full cover, fire, and then sidestep back behind the cover: no snapfiring and no modifiers.
A question is whether the defensive bonus for snapfiring (from behind a low obstacle) "stacks" with the defensive bonus for being behind the low obstacle itself? A Predator snapfiring from behind a low obstacle would be at -3 (!) to be hit while snapfiring. You would need half a dozen guys covering 0 at medium to have about a 50/50 chance of taking it out.