I guess "must" is pretty strong. In the current environment where all the attention is focussed on the spammers and not the people using the spammers' services, there's no incentive for the users to hide the money trail. So the chances are that if you get a spam for Bill's Business, Inc., and it's not a frame-up job, then there will be an entry in Bill's accounts saying, "Spamming Run: $300". If it was payed for in cash, you then lean on Bill. If it was paid for by traceable means, e.g. wire transfer, credit card, etc. then you follow the trail. If the trail goes outside your jurisdiction, you can always go back and lean on Bill.

Yes, it's imperfect. However, it's got a better chance of succeeding than simply charging people to use email at all.