I don't think it has ANYTHING to do with insurance, since the kids can use the gym equipment as part of the clubs. But that's only for an hour, twice during the 7 day. And then those times were at night, and conflicted with our late seating dinner, and he wanted to eat with us.
My 17 year old DS was VERY disappointed since he wanted to do some conditioning to get ready for Varsity Baseball.
This is certainly something Disney needs to fix. It is a FAMILY cruise afterall.