I think the purpose is that they understand you buy a license, and at some point might have a reason to transfer it (new site owner, another company, you move it on to another person).
For free, as an exception, they permit you to transfer it.
But to avoid users buying up licenses in bulk and transferring them to make a buck, or whatever their reason might be, they limit transfers to incidental. Rather than creating the option for extra overhead for themselves.
Of course, this might change in the future.
I've had positive experiences with having licenses transferred to me, and away from me.