SWsoft's Virtuozzo has been around for a little over 6 years I think... and OpenVZ is the
GPL'ed release of the kernel code and some of the userland code... and documentation, etc.
Linux-VServer has been around for as long if not longer. Much of the code that is being
adapted into the Linux kernel regarding control groups (trying to get used to the new name)...
has been contributed by Google, IBM, OpenVZ and others... working together to get a consensus.
If they wanted to borrow from FreeBSD, they'd probably be using FreeBSD. My guess is that
Linux and FreeBSD are different enough that adapting the existing FreeBSD jails code would
take as much work if not more as adapting any other project's code would... and having
everyone work together will hopefully lead to situation where everyone is happier than they
would have been if a single, canned solution were chosen. It also gives them the opportunity
to take what they've already done, and improve it yet again.