I'm not stating an opinion either way, but since you seem sincerely confused about why a 15-YO would be a problem on this, I'll try to clarify. I think the issue is that
DCL likes to reserve some activities for adults only as a benefit to the adults. The same way some people don't like it if a teen is in the bar after 9PM to watch a musician, even if they are "quiet and well behaved," or during adults only trivia, some people feel uncomfortable with teens on the tour even if the parents are sure they won't get bored or be disruptive. Likewise, some people feel uncomfortable if they have to speak up and say they'd prefer to keep the adults only activity, for adults only, so they rely on the CMs to enforce the rules.