I think it has more to do with tradition than anything else. Salties have been cycling tanks with shrimp since before the whole ammonia thing got started in FW. And a lot of them aren't going to change their way of doing things now…
Ammonia from a bottle isn't adding in phosphates. If you get straight ammonia with water (no detergents), it doesn't have anything more potentially harmful than a hunk of rotting flesh. The nitrogen-eaters aren't the only ones coming and setting up shop on that carcass. After two or three weeks, when its good and ripe, would you take it out and eat it? If not, then what did you add to the tank? Would you take a glass of that water for drinking?
It seems mighty strange to be adding ammonia to a fishtank, but it works and its safe (at the right dosage).