Re: That's Why...
Firstly, we need to differentiate between Linux the kernel and Linux distributions.
Linux the kernel has the ability to accommodate loadable modules some of which are included in the kernel source. ZFS isn't one of these. Neither are such items as user space file systems and binary drivers.
Linux distributions package the kernel, a bundle of tools including a whole raft of more or less Unix-like tools. As we've seen Ubuntu bundle a GPL'd kernel-derived module which interfaces with the non-GPL'd ZFS file system. Ubuntu, presumably on the basis of legal advice, take the position that this indirect link between the kernel and non-GPL code isn't such as to make the ZFS code a derivative on the kernel and hence isn't subject to the GPL.
Secondly we need to consider the legality of this in terms of licensing. As I've spelled out here a few times whenever some new legal situation arises the only way to be sure of the way the law will deal with it is the decision of the highest court that adjudicates (which might differ in different jurisdictions). The only way this is going to receive such a ruling is if someone with suitable standing, presumably a kernel contributor with contributions to the parts of the kernel most closely associated with the interfaces being used, takes the case to court. RMS might not like the situation but he isn't a court and unless he takes action himself and convinces a court that he has sufficient standing then there's not a lot he can do about it AFAICS. If the kernel contributors don't take legal action within a reasonable period of time we have to assume that either they accept the Ubuntu position, take the view that they don't believe that they have a strong enough case against Ubuntu or simply can't be bothered.
It's worth noting that although ZFS isn't GPL it is, AFAIK open source under the CDDL licence. This is in clear distinction to many binary-only drivers that many Linux distributions include. If the inclusion of ZFS were successfully challenged in court distributions might start to be concerned about including such drivers; under such circumstances the entire Linux project would be in serious trouble.