I'm not going to talk about the campaign manager aspect since I have no idea on how they (the campaign managers ) select those that will participate in any campaign that's being managed by them.

If you take a good look at the forum, I see it as a historical place where you have several topics, discussion on different subject matter and sometimes when a newbie comes to the forum they are always advised to go through previous threads to learn more om certain issues or topics, but if you as a forum member that's supposed to contribute to the already existing knowledge or opinion in the forum decides that you don't what to instead you want to delete your posts (that could have helped a newbie or someone in search of such knowledge in the future) just so that your account stats can look cool and admirable, you're not only cheating yourself but also the potential of the forum. Sometimes we go back to our account post history just keep scanning through them to see the growth that we have been able to achieve since the time we came to the forum.

Another reason could be that they are deleting some posts they feel are spam maybe during their early days or due to the amount of knowledge they had during the time they made that post.

But if they are doing it to cheat their way into a campaign and you're very sure of it, then you can simply notify the campaign manager of it and if they find it unethical they can remove them from their campaign but if they're cool with it then we all move on with it. Just my 2 cents.