Absolutely right, but it isn't all about people dodging the system and working illegally. From what I see, the vast majority of people doing such business would actually like to do it legally and above board but know it is as good as impossible. These people slip into the huge cash economy that operates here because there is no viable way to do it legitimately. To do things legally makes them visible and it does appear targeted by bureaucracy and endless rules and regulations.
The net result is that they don't bother, they make more money and Turkey once again misses out on the taxes and genuine employment opportunities. British people have a lot to offer in most industries here, and when making money for themselves could be making money for Turkish partners, the state (in taxes) and employment opportunities for many good local people. There is little or no incentive for many to do that.
My opinion has long been that the work permit system is counter productive for Turkey as a whole, and hugely closed market for a country that apparently wants to join the EU. You can legally run a business but you can't work? What the hell does that mean? Who does that help? A work permit system (if it is insisted upon at all) could be put in place in a couple of hours where an individual can have one, dependent on them employing at least one Turkish national at a sensible/fair salary and paying their SSK/National Insurance. While they pay those, ther work permit is valid. 30 days without paying both (for the British person and the Turkish employee) and the permit is repealed.
Result? More local employment, more taxes for the Government, more wealth being generated and spent in the country - it's all positives, there is no down side. At the moment we have a hidden batch of cottage industries, making money for the individual Brit only and keeping their heads down. I know the resentment of local Turkish businesses - a classic example is the airport transfer companies - the legitimate Turkish businesses see Brits at the airport picking up, knowing full well they didn't have to pay taxes, correct insurance and for TURSAB membership. Of course it would make you angry. Unfortunately my whole point is that they aren't dodging the system because they WANT to, it's because they have no other option.
Bit long that, sorry