I already PM'ed you about this .
First of all, its probably not a good idea to flaunt one of the rules and then call them "nazis" unless you want the banhammer! A lot of others forums have very similar rules, so its not just TF being particularly strict. There are places that ban all discussion on CP trades and sales to keep discussion focused intellectually.
In any case, the rule isn't to force members to only buy plants from the shop or anything like that. If I recall correctly, its because there's a lot of unregistered people and people just starting off that browse the forums/chatbox and sort of just lurk. If we were allowed to discuss vendors, it would drive away a lot of these people from buying plants at the forum store.
Furthermore, in places where discussing vendors is allowed, there is a tendency for people to give bad reviews to certain vendors because of some incident, and the fans of said vendor then start arguing back, resulting in large conflicts.