There are many reasons why people delete their post, I was asked to clean my post history during my transition stage (from a spammer to a quality poster) and I deleted closed to 500 posts, I don't know what the reasons are for those you observed deleting their posts but we can't just assumed they're trying to cheat the system and also there's nothing they gain from campaign campaign managers when they delete their post. It doesn't increase their merit or post quality so I don't see why this is cause for concern. They're actuator doing the forum a good thing because it's less work for moderators to start deleting their posts when they get reported.

When they delete their posts it equals to less work for spam busters so they're actually doing the forum more help than harm but as they're deleting, their next posts should have better qualities. You should be less concerned about this as managers don't look at  how many posts you have instead they look at the quality of your posts, and to some extend having high quality number of posts could get you into campaign when your merits isn't high as others. Managers will prefer posters that can make more posts than those making just the minimum needed to be paid weekly.  More post means more promotion but obviously they have to be quality posts.