Leon wrote in
:

Gotcha. And you should never have an issue with the nut coming off by
itself, arbor nuts are self tightening with use.
On the flip side of that, I have never oiled the arbor nut on any of
my saws and never had a problem with removing the nuts.

I would say that's an expected consequence of regular use.
If you routinely remove the nut (to switch from crosscut
to rip or dado set, etc) then corrosion and gunk never
collect on the threads, so they work as intended.

In the OP's case (the post from 2010) I'd guess Ryobi uses
air wrenches in their factory, and likely the nut was crossed
and the assembler just ran it up that way, not being able to
feel it wasn't right.

John