I agree with the limitations of engagement on snipe. It also makes sense thematically. As long as the squad doesn't have anything to deal with it can snipe from range. But when engaged they are caught in a dog fight, they can't line up a shot.
I agree with you about how it should work thematically but it is not supported by the rules or at least I didn't see how yet.
About its balance I don't know. The new wave don't arrive until this Friday here. We have this thread about how snipe should or shouldn't work, we have threads about defender spam and cloak. But I didn't see a thread talking about how awesome or disappointing snipe is.
Played with it a couple times, Snipe is pretty good.