It amazes me how many posts you see from people worried about coming to this part of the world simply because Turkey is next door to Syria, and even consider cancelling their holidays, yet then come and hire these machines when there are so many accidents.
We were up by Doctor Tolga's in Hisaronu a few days ago when a young girl was pretty much carried in by her friends, she'd come off one and had cuts/grazes all down her arm and leg on one side. (Fortunately she was more panicked and covered in blood than seriously hurt.) No helmet, had just thought it would be a laugh to hire one.....
Also read a post recently from someone whose uncle (I think) had used a false licence to hire one as he didn't hold the correct licence himself and was then incensed after it ended up over a cliff (luckily he didn't) and the hire company insisted he pay for the quad bike before they'd release his passport, which it seems they took presumably for some kind of security.
I don't know if this is the same case you mentioned,Fabian, but in my view they were perfectly entitled to expect the guy to pay for the quad as he'd given false information to hire it!