If the game suddenly got more adult, then I would lose the option to share it with my kids. Teens would lose the option to buy the game from a store without having an adult present and approving the transaction.
"Just switch it off" is a lot harder than it sounds, but it's an option I see offered up a lot when some players want a certain feature and others don't. At the end of the day, you can't add everything. It's just not practical to write hundreds of different scenarios for "if X is on, this happens" and "if X is off, this happens."
At some point, the creators have to decide what they want to be part of the game and what they don't. They decided on the teen rating for this right from the beginning. They stand to alienate a much larger chunk of their audience if they change the rating than if they don't.
