As a recipient:
I pay for the trash bags to put the junk mail in
I pay for the trash service to take the junk mail away
I pay with my time having to sift through the junk mail to make sure my real mail hasn't been inserted into it
I pay in time when I have to track down a bill that's been thrown away when it's stuff into junk mail
Regarding hijacking servers, that's illegal and it focuses on a specific action. Charge him with hijacking a server.
The fact is, as one another person posted, the reason junk-snail mail has not been outlawed is because it server an economic purpose by feeding money into the postal system. That's why one in illegal and the other is not. IT's not based on right or wrong, it's based on economics and who benefits from it being legal. And that thinking corrupts the law.
Why should I be forced to receive something that I didn't ask for? Whether that be spam or junk-mail? Making one illegal an the other illegal creates ambiguity in the law because the law is no longer about right and wrong. Spam vs. Junk-mail just being a small example.