There's no reason why it has to be defined this way, though. Create a new category for "marketing mail" (or whatever), define what belongs to it, and require senders to continue to presort it, but make it cost 10x as much (or whatever).
This is never going to be perfect, of course, but anything that reduces the amount of crap I get in my mailbox every day isn't just great for my levels of annoyance, but also great for the environment. 95% of what shows up in my mailbox ends up in the recycling bin. Creating, printing, shipping, and recycling that material is a huge waste.
It's certainly possible to do that, but keep in mind that if the government (or an organ of the government) is in the business of deciding what category of speech something is, that is eventually going to cause a problem.
This is never going to be perfect, of course, but anything that reduces the amount of crap I get in my mailbox every day isn't just great for my levels of annoyance, but also great for the environment. 95% of what shows up in my mailbox ends up in the recycling bin. Creating, printing, shipping, and recycling that material is a huge waste.