Put simply, it allows for a dumb down/streamlined gameplay that promotes micro transactions/player engagement.
Previous halos had issues with betrayers. Instead of implementing a effective ban or vote to kick option they straight up removed friendly fire altogether.
By doing this it increases player engagement… Thus more micro transactions sold. Notice how friendly fire is on in competitive modes; that is because comp players don’t mind it, as it adds to the competitive nature of ranked.
But god forbid player engagement is reduced in big team battle or social slayer. It’s all about money guys, companies specifically higher people to develop the best monetization systems, these systems can and do effect gameplay. If this means changing the entire make up of a game to increase player engagement they will.