I think you are getting hung up in the "business" aspects that have been brought up. Also, let me put out there that I don't necessarily think banning is the correct decision, but it is a reasonable decision.
Lets say I started a free book club that meets every Wednesday at the local library. It proves very popular, and next thing you know there are dozens of book clubs just like mine around town meeting at the exact same time on the exact same day. Some people like those other book clubs so they leave mine & join those - this is just the way of the world. But if people from the other book clubs started showing up at my book club with a hand full of promotional flyers to pass out to my book club's members, I don't think it would be unreasonable to ban that practice.
At one point, I think the Facebook groups were exactly what you experienced. An organic evolution of the dismeets that added to the dismeet experience. But as several people have mentioned, now often once the dismeet thread is established, someone has posts within the first few threads an invite to the FB meet page that started even before the dimeet thread began.
Now lets muddy the water a little more with the fact there is a "Disney Cruising" facebook page that maintains links to all of these facebook cruise pages & encourages its members to find the link to their cruise's page. (There is apparently some debate as to what level of involvement a competitor TA has with the "disney cruising" FB page - so I wont even get into that side of it) But again, leaving out the "Business" aspects, and just focusing on two communities of
DCL fans, I don't think it is unreasonable Disboards bans these facebook groups from promoting themselves on Dismeet threads. May not fix the problem - facebook meet pages may already be 100 times more popular than dismeets these days - but I don't think the ban is unreasonable.