The pay rate is not the problem, even those in high paying campaigns also spam. The problem arises when some campaign managers in both low paying and high paying campaigns set very high weekly post quotas. When campaign participants are expected to make over twenty posts and even more in a week, many focus will shift from quality to the urge to just meet up with the target for the week so they'll be paid and retained in the campaign too. Some quality posters too are guilty of this. When campaign participants write under pressure, the post might lose quality.
If post quotas will be reduced to a reasonable number, pressure on participants will reduce too and ofcourse there will be a clear difference in the post quality because participants will now be focused and intentional.
That's certainly true. I've seen quality posters in "lower" paying campaigns, but I've also seen low quality posters in better paying campaigns. The reward/pay doesn't always determine the quality, but I agree to some degree that the higher paying the campaign, the higher requirements and standards the manager and the advertiser is going to require. I personally believe that the higher the posting quota is, the more spam is created. I bet all of us remember a specific gambling campaign that allowed up to 60 or 70 posts per week. God, the mass spam it created.