I've done some extensive research on this, and the simple answer is that the Amiga's parallel port (on the 1000 and 500) don't support enough control signals to be able to run 95% of the parallel to SCSI adapters that were designed for PCs.
Oh, so that means A1000/A500 don't use a standard parallel port? That's a shame since I was thinking of getting a parallel port CD-ROM.
I thought that if it was connector compatible all that would be needed would be a device driver.