I don't see this as a big issue. It is just simple. If I am out of a campaign and a new campaign is launched, while applying, I will definitely wear the signature and avatar such that if I'm accepted, I don't get to bother myself anymore.
However, if you are in a campaign already and wants to switch to a new campaign, there's no need removing your paid signature and avatar when you aren't accountable yet in the new campaign.
...this is possibly because there are alot of cheaters in bounty campaigns where if the manager accept a participant while he or she is not yet wearing the required signature and avatar, they may end up not wearing it.
LOL, it's not making much sense. If this can happen and the manager doesn't notice, it means that after being accepted in the campaign, the participant can still remove signature and avatar and still gets paid.