That's a really good point that DTrace is Free Software. I just now eventually came around to realizing the relevance of that in another post on this thread.
However, my point here was that resulting restrictions on use and redistribution are similar:
The copyright holder on DTrace has granted permission to use it in Linux systems, but the GPL which governs redistribution of Linux source code forbids redistributing derived works which include non-GPL'ed code such as DTrace.
This winds up imposing the same sort of restriction on users and distributors that a proprietary driver does: the copyright holder of the proprietary driver grants you the right to use it in your Linux kernel, but the Linux licence forbids you to redistribute it (assuming the current standard interpretation of GPL applied to this issue).