Who would sue, and what damages would they demonstrate in this case? Both the Linux and ZFS source is available and you are allowed and encouraged to build and use them. Redistributing binaries can be thought of caching precompiled artifacts, merely speeding up what you can already do manually (and with reproductible builds the result should be identical). Nobody loses anything.

Let's say I built a magical compiler capable of compiling a Linux+ZFS kernel in miliseconds. I put it behind a web UI which accepts a Linux tarball and ZFS tarball and spits out the compiled kernel. Because of some mysterious bug I am still trying to solve, only specific Linux and ZFS tarballs work, so I put validation to hash the uploaded tarballs and only let through the ones that are known to work.

Let's peel back the curtain: there is no magical compiler and all this is doing is hashing the input kernel & ZFS source tarballs, using it as a lookup table in a cache of precompiled binaries, and spitting out the matching one, which is currently not allowed. But let's assume the compiler was real, in this case it should be fine, even though the functionality of the system is no different.

I would at least understand if one of the licenses explicitly restricted the right to distribute ready-to-use binaries as a way to carve themselves a moat so the authors are the only ones that would be able to do it (and thus charge for it). But that's not the case here, nobody is better (or worse) off whether I waste time building it manually vs reuse someone else's earlier effort of building it.