Why should I tip a contractor setting their own rates?

I've gone on a couple of rants about the debate we have in society over the custom of tipping. It seems like everywhere we go, we're expected to pay for the product or service AND pay more to show appreciation for it.

My perspective shifts dramatically when the service provider is an independent contractor. This is where I find myself diverging from common practice, forming what some might consider an unpopular opinion. When I’m visiting a hairstylist who works out of their own home, for example, or a mechanic in his own garage, my approach to tipping changes entirely.