IANAL, but Turkcell would lose the case in Turkey too. This is not due to net neutrality regulations (Turkey deliberately lacks it), but due to case law arisen from competition and customer rights regulations. However, telcos work around that too, by "leasing" modems, like telephone divisions did in the past. Does the trick of "leasing" work in the EU too?
in marketing they try hard to make it sound like what you are going to get by renting their device is WiFi not just the ability to turn on WiFi functionality of the CPE. of course everybody wants that but most people don't get that's not something that has to be provided by the ISP. I am not sure if its required, but i have seen often a lower end device (without WiFi accessible) is given for the lifetime of the contract free of charge.
in Germany you have the right to use a compatible device you own yourself. However my ISP Vodafone does not accept lots of modems as compatible and when this regulation started there were basically none you could actually buy. Its not much better now i guess but i distress.
EDIT:
reading your comment again the trick you mentioned probably works because its "yours" when you lease it instead of renting it?
