I agree that a lot of smaller kids cause the lifeguards to jump in...however, I do think they're very concerned about the height limit too. The whole setup would be easy(ier) for a person to go too close to the wall if they were too tall. Also - the pool is only a certain depth. I think there is a real danger for a taller child/person getting close to the bottom upon the jump part.
If they weren't concerned about it - I would be on that zip-line in a heartbeat. The most fun thing at BB, IMO.
I do think they should have a height requirement too. I stood watching kids that were too small to reach the handles for a long time in August. It was pretty dangerous as they would go near the edge and try reaching up and come close to falling off the side. Then they would just look around and sometimes a bigger child (presumably a sibling) would try boosting them up (again precariously). Not to mention this would hold up the line endlessly.
I think they should have a height requirement AND a height limit.