Maybe this argument is less useful for BSD users, but wouldn't most users be using the stock kernel? So it's up to the whims of the distributors whether to include it or not.

It's pretty common in the BSD world to build your own kernel. The entire build system, imo, is much more streamlined and well thought out than linux, so relying on the distributor is not needed. Also, one of the missions of NetBSD is to get it working on everything, so as others have stated, being able to poke at the kernel in lua, both for developing drivers, and for pulling state out of the kernel for general debugging, is very useful.