Stunna pays for posts because that's essentially one ad impression of his site being put out there and the exposure it gets to new users. Activity is irrelevant really aside from the size of the signature, but the size is irrelevant if you're not posting, so it's still essentially the volume of posts that count.
And we can all speculate and suggest payment on the prices paid but the ones suggested so far seem very biased without taking into consideration how this is worth it to Stunna. It's easy to suggest people over 120 activity should get 0.2 or 200 gets you 0.3 when you have over that amount and would rather just get paid for that than posting. At 0.3 it would've taken you 1,500 posts to do that before, which is a lot of work but was worth it to Stunna, but now you essentially get it for free and Stunna could potentially get nothing or most probably a handful of posts for his money.
And wether Stunna pays per post or not is largely irrelevant as there are other deals that do. So unless paying by posts is actually banned people will just move onto another deal if this one isn't beneficial to them and any damage to the community on these deals by spammers will continue. The only other deals I've seen before paying like this system offer peanuts because that's all it's worth, not to mention they're usually exploited, so that's why I'm really intrigued and interested to see what Stunna actually offers because I don't think the numbers will be anywhere near what has been biasedly and wishfully suggested here so far.
If 400 posts a month is not constituted as close to spamming then that's extending into the questionable range
Productive posting can be done at that level but its much more difficult to find those types of users who will post daily on a consistent basis
That's around 13 posts a day and seems like a fair level between advertising and spamming not those who usually don't or would rather not make many posts.
I would say that this system doesn't just account for present posting but also past posts if the idea of 1 post is equal to 1 ad is true then the same extension must be given to past posts someone with 1500-2000 past impressions aka posts certainly qualifies to be in their respective tier as long as their activity remains in the same range, since the chances someone will see old posts may be smaller but still exists because it is an accumulated presence over time.
If someone has 10000 posts its not the same if they were here 3 years and did it or if it was done in 5 months only 7 Users are at that level all of them are Hero not to pick on you but just as a point of reference, the average user won't pass 400 a month and even chartbuddy an automated chart script only posts around 720 times a month or around 24 posts a day one an hour.
https://bitcointalk.org/index.php?action=mlist;sort=posts;start=0
I wouldn't say that being unfair to activity is a bad thing it just means you were here long enough and produced productive discussion, the admins and the meta thread is all about trying to find that balance.
I see what you mean if you hit the top tier as it would reduce incentives to post, on the other hand if they were here that long their is reason to believe that they will still participate productively to the forum it is not a 0 sum game. It's easy enough to keep a minimum along with the activity based tier at any level its an easy modification to the system above. Min 50 posts payout ranges based on activity and 400 posts upper range a month. It does not need to be a fixed amount either but it could be a mixture based on both.
As for individual users they may move to other signature campaigns but it is worth noting that the most successful signature campaign on the forum was coinlenders and inputs.io not Primedice which came after their wallets got hacked, and people needed to move to another service.
That said Tradefortresses program was a bit unique and no one else imitated that model possibly because of the concerns you mentioned, still with a bit of tinkering it likely would work pretty well.
It really just depends on how you manage it, since some people can be productive and post a lot there could be exceptions but typically most users won't fit into that category.
As for Size (Size Matters in this case )
&_&
You guys and your dirty minds I know the demographics here XD
https://bitcointalksearch.org/topic/demographics-of-bitcointalk-100294