Obviously haven't read through everything, but I think their policy is fair. After all, you don't really want to link to hundreds of third party sites which have whatever draconian terms they require users to agree to, to use their stuff. Here, the definition of "free" becomes really loose (eg, "free if you make 100 posts"), so by sticking to the mods section, you essentially make it clear what "free" refers to.
Makes it harder for people who make sh*t plugins to use it as an excuse to promote their equally sh*t website. Fair game to me.
If you really do want people to visit your website, then simply don't list the plugins on the mods site. Easy as pie, just like I'm doing here. If you've got good stuff, people will come anyway. If all you have is crap, well I'm sure the community won't sympathise with you.
If the reason you write plugins is primarily to increase e-penis size (eg "look at me! I can write plugins, so I > you!") and broadcast that to as many people as possible, then I don't see why MyBB should bother supporting you. On the other hand, if your aim is to help the community, further your own experience, or the like, then you should not care so much about how many people use your code.
^ Having said that, I bet the majority of developers do want people using their stuff to signify that their time was worth the waste.
I am using currently
1. Ads after first post
2. Dynamic metas
3. Nofollow
4. Pm on registeration
5. Revenue sharing
6. Seo links
7. Youtube bb code
8. Subforums in columns
9. Contact form