So, if the parent had a child with them when they purchased the game, it must have been FOR the child. I guess if you buy beer at the supermarket with the kid in tow, it's for them too.
We have a lot of PS3 games that our kids are not allowed to play (and that the adults aren't allowed to play until after the kids go to bed).
Like the contents of the liquor cabinet and the gun safe, they know it's there, and that they're not allowed to use it (and, more importantly, WHY). Every household with children contains things that are not for children. A big part of parenting is teaching your kids to recognize and avoid the things that can hurt them (without turning them into the forbidden fruit).