They didn't "let" them. It was something that the game let you do by design whether they intended for them to be able to do it or not. The prime engram exploit isn't cheating. You just figured out a way to get more because all they were doing was literally just playing the game. As for glitching into the raid, people have been glitching into these areas since the beginning of D1 and it'll continue to happen as Bungie can't check every square inch of map.