Because they didn't like people doing it, because they want people to kill him with bullets! Pushers didn't hack the game in any way, shape or form they exploited the bosses/boss arenas mechanics. The Raids are Bungies golden ticket in Destiny, they're proud of them, people pushing the hardest boss off the edge made their game design look stupid, they didn't want to look stupid so they put 'baby bumpers in' yet people still push him off. They designed the boss fight badly by allowing pit falls to be part of the arena, all people did was use them to their advantage.