A person's reputation is not what is in their signature line, it is how they act and what they say. Having a paid signature is nothing more then receiving money for advertising a good or a service. If you are against advertisements then you should be against this forum (that sells ads) and any source of media/news as they make almost all their money from advertisers.
Yes, a person's reputation is based upon what they say. When you *sign* your posts with an ad for some product or service, you are *saying* that you endorse said product or service.
I would disagree that people are "signing" their posts with their signature. They are simply "wearing" their signature in exchange for payment. This is the exact same arrangement that theymos (via the forum) has with a number of advertisers (see
this post. Unless you have ads disabled you will be able to see these ads after the first post in each page, you will also see a disclaimer that advertisements are not endorsed by the forum.
This is also a very similar arrangement as to how newspapers and TV stations make their money....via advertisements, I would say that common sense would dictate that advertisements are not endorsed by these channels of information
EDIT: unless you personally endorse something you are not "signing" your messages with a specific advertiser