Well, I don't think there's anything wrong in applying a signature code on your profile when you aren't in a campaign already. Yes, that's some type of free advertisement but it's something good from participants side who apply to get accepted in those campaigns as it doesn't harm them in any way or doesn't cost them a dime.

It's a type of good will I believe and managers do admire such participants because they understand that such participants are active members of the forum and can be helpful to promote a service via their signature space. I also believe that something like this shouldn't be mandatory but there's no harm if someone applies a signature before getting accepted in the campaign.