Perhaps it's because the name brand is spelled in all caps.

Actually, it is. It stands for "Stupid Pointless Annoying Messages".

...At least, that's what I've always thought it to be, anyway. Now that I go back and check, I can't quite find a source for this claim.

My understanding is that it's derived from 1970s database programmers "spamming the database" with dummy data. The original term comes from Monty Python's Spamalot skit, I believe.