[boost] intrusive_ptr design question

I was wondering from a design perspective, why was it decided that intrusive_ptr should rely on user-defined free functions intrusive_ptr_add_ref and intrusive_ptr_release to handle the reference counting? This makes it somewhat inconvenient, for some constant type T, to support having two different instances of intrusive_ptr<T>, each of which use a different reference counting strategy. If there's a clever way to allow this that I'm missing please advise.