Why not the stick, instead of the carrot? Make any bulk mailer that doesn't follow the rules guilty of abusing federal communications channels, similar to how we prosecute people who are misusing radio spectrum today.

We make things corporations do illegal all the time; it doesn't hurt corporate feelings, unlike restricting the rights of individuals. (And individuals, as far as I know, don't bulk-mail.) What would be the downside to this?

I think most people would be fine if there was a 5% discount for bulk mail, however they cut it way more than that.