The problem is the incentive for unknowledgeable people to post as much useless drivel and inane nonsense as they can to increase their post count (and therefore their income).
One of the issues - as already mentioned - is that sometimes it's not easily classified as useless drivel or inane nonsense.
But based on analysis of a few users who I saw posting repeatedly in threads I follow, it is rather obvious that they post for signature sake.
One example: the entire week before joining a particular signature campaign: 0-7 posts per day, median of 2. On the day of joining: 67 posts. The next day: 19 posts. total post count for 16 days following that registration: 227 posts. After that? A whole bunch of days with zero of few posts. This is, sadly, fairly indicative for most people who are very active with signature campaigns. You can quite literally grab a user's post history, plot a graph of number of posts per day, point to peaks and suggest that this is where they signed up for a campaign, and more often than not you'd be right.
From reading some actual posts, it's also sad to see how well aware they were of having to play by the rules, that signature campaign managers supposedly check all the posts to make sure it's not spammy ... and thus knowing that they can't just post "lol", and have to at least write something that seems like it might make sense in the thread, even if it adds nothing.
Literally, one of those posts reads:
Going to improve the quality of my posts now that so many people are being banned for spamming
This is sad. The quality of one's posts shouldn't be for fear of getting banned.
Nor should the reason for posting be in order to participate in a signature campaign. From a campaign manager:
You were on pending because your post history is not very good [referring to low number of posts].
Of course it's advertising, so they want you to post as much as possible, yet at the same time they want you to not be spamming for fear of themselves being shut down. These two goals are already at odds with each other.
Moreover, many of them understand that higher level members - say, legendary - may get higher payout.. so not only are they posting in order to get more payout, they're also working to get into a position to get a higher payout.
At the same time, that 'legendary' status can land them other perks (forum and offers), so they're getting rewarded left right and center for this behavior.
I'm using the signature blocking script, and that at least blocks or lessens the eyesore - unfortunately this doesn't help against the collateral damage. Ignoring users altogether is another approach, but there are sadly good people making good posts also filling out their signature campaign requirements that would be caught in that net.