DannoXYZ, it probably isn't. Sounds like it was indeed one of those cases where the freewheel was designed for a hub that had a longer threaded portion, and thus a spacer is required.
If you need to just keep the freewheel from being overtightened on the hub, another trick is to thread the freewheel on, then fill the gap between the cogs and the hub by wrapping fishing line or something similarly tough and thin around the hub.
This is an incredibly bad idea for reasons which should be obvious.