It isn't. There's a crawling animation for both 1p and 3p, it just snaps to the firing animation the moment you pull the trigger. It's the reason there isn't a lowered weapon stance for the standing animations, because there isn't an actual animation for aiming down the sights transition (it's simply done by the engine). It isn't actually much of a problem for crawling as you want to stay still while firing otherwise you'll get massive amount of deviation and won't hit the barn doors.