Actually.... its a co-ownership arrangement between the contributor and the project...and not a straight up assignment of all rights to a single entity. So its a little more complicated a situation than just a straight copyright assignment because now multiple parties have the ability to re-license a particular contribution. Though I think your right in that another corporate entity would avoid contributing manpower to even if there was some marginal benefit to the rights maintained in a co-owership arrangement.
And actually... I would image violation enforcement actually gets much more complicated with co-ownership of a particular contribution because you'd have to check with all co-owners to make sure they haven't potentially licensed the work to the alleged violator. Even the more typical argument for assignment to a benevolent project entity to make enforcement easier doesn't necessarily hold with co-ownership.