[mrtksn]

AFAIK foreign IMEI was never illegal in Turkey. The registration requirement started in 2011, before that you could just pop a local SIM and use it.

Any IMEI can be registered within a specific timeframe(they change it from time to time but it's usually 3 weeks to 12 months) or the GSM operator will deny service to your device. It could be that you missed your deadline because to register an IMEI you need to do it within certain timeframe after you enter the country, there was even a business of paying tourist to use their passports for registering IMEI of black market imports.

Those who legally import phones for sale in Turkey also register the IMEI of every device but at different and much lower rates than the individual registrations(they pay other taxes).

[throw3849]

Interesting, this was 2014, perhaps clerk did it to save me money for device registration.

Now I just use my EU data plan, it has enough data for short stays.

[mrtksn]

That's probably it. Instead of you paying this ridiculous registration fee, the clerk probably offered you a cloned Turkish IMEI. That's another popular workaround!