One key difference is that junk mail doesn't take up much, at least as far as resources go... On the other hand, as a network administrator, I find that I have to spend at least a half hour a day entering domains into our email server's "refuse" list...
And don't even get me started about spammers who use other servers for relay, in order to hide their identity!
Mark
Have you ever considered using SPEWS (Spam Prevention Early Warning System)? They regularly publish lists of known spamhausen in several ways including Procmail filters, router DENY tables, and records you can use as DNS zone files. SPEWS is quite effective at helping you shunt off much of the spam before you receive it.
I'm not SPEWS and don't represent them, but my company uses their service and is happy with it.