For some folks, perhaps they just want to promote their local hobby shop, so they put the name in their sig. For others, sponsorship is as much about brand ambassadorship, as it is driving.
There is typically a multi-tier sponsorship arrangement. The "fast guys" are the ones with full factory sponsors. They generally don't have to pay for their equipment, and may even be paid to race (think Drake, Lutz, etc...)
The "B-Team" is usually the local /regional fast, or above average guys, with a lower level of sponsorship (25-75%). We pay for all of our stuff, but get a discount as part of the deal. Our responsibilities are to promote the brand, provide product information to racers, answer questions, get visibility at the local / regional level, provide feedback to our sponsors regarding how the product is being used, etc... Basically, our job is to support the efforts of the "A-Team" guys, since they can't be everywhere at once.
So, to answer your question, judging a person's sponsorship by isn't always accurate. Mine, for example, lists ToyTown Hobby shop. I'm not sponsored by them, but I still like to advertise their name. Mainly because I want to support my local shop, and the owners are friends of mine.