i know there is no rule restricting users from deleting their previous posts when they feel like it, but I don't know why I feel doing such is a cheating on other honest forum members, as well as campaign managers who might believe you to be a quality poster based on your low posts count and merit earned, not knowing that you are just an average poster like most users of the forum, that deleting previous post is what has kept your post counts down..
To be honest, I don't think that they are fooling anyone other than themselves if they think by deleting posts and having better merit/post ratio will get them into better paid signature campaigns. Likewise, any manager that can get fooled by that is not a good manager at all and is not checking post history properly.
Another reason why some members might do that is because they could be embarassed by their early posts when they just joined the forum but its hard to say without you sharing the names of those members wo we can see type of posts they were deleting.