Please don't quote me on this. I think it's not a License question but they're taking a different approach. Linux binary compatibility has been around for a while now, at least longer than modern LX zones (I don't know about old Solaris lx zones).

There are some more subtle differences like the for BSD Linux emulation is a global setting and 'lx jails' are just jails w/ a Linux userland while branded zones are special kind of zones.