If signature campaigns see value in Off Topic posting why shouldn't they pay for it?
Regarding posting limits, any action like that would be fruitless and easy to circumvent in this environment, where account trading and multiple identities are acceptable.
As for the post restriction, I don't really believe it'll...completely be circumvented. Perhaps a little. Not too much. I assume most average people here only have maybe 2 or 3 alternate accounts. I have yet to even make my alternate accounts, but I don't know if it's a necessity to have several. I don't really believe in a post restriction, but if it could quite possibly stop spammers, it could be taken into consideration.
Campaigns don't really care about the posts themselves, most of them don't allow spammy posts because they know mods wont allow it. At the end of the day what matter is the signature, no matter how short or non-constructive a post is the sig for their site will still show and the more posts the more places it will show.
I'm surprised they don't pay more if you have already a lot of posts since everytime you change your signature it will change on all your previous posts aswell. Im sure the campaign signatures would not care a bit if their user spammed and mods wouldn't ban them and really the off-topic section might be the best section to promote something, everyone can check it out meanwhile not everyone will go to the gambling section.