It's not a bug I think, it's just the way it was programmed
This is true, but it was a bad design decision. It makes zero ammo + nonzero delay act exactly the same as zero ammo + zero delay, meaning a particular combination of scheme values is completely useless whereas without this, it would be useful.
So I will remove this limitation in the next version of W:A.
There's another related bug/flaw. When a weapon's delay becomes zero, it takes another turn before it can fall in a crate. This creates a discontinuity between delay=0 and delay=1; something with delay=0 can fall on the first turn, but something with delay=1 can't fall until the third turn.
(for example, infinite donkeys with a 255 delay result in a ban of donkey crates, even if you get them).
This is of course only useful for superweapons, because you currently can't set their crate probability.
A 128..255 value is actually interpreted as -128..-1 respectively, and negative delays are specifically treated by the game as infinite delays (so even after 65536 turns or more it'd still be delayed).
While it's true that you wouldn't be able to use an infinitely delayed weapon even if you collected it in a crate, the game disallows delayed weapons from falling in crates. The only way to collect one would be to collect a Crate Shower crate first.