When I say screwing over, I don't mean that as a value judgement on Greece's position. Greece's position makes sense, international law is on their side as is history. I mean that it objectively is a bad deal for Turkey, I'm sure you agree with that.
i.e. if anything north of Antalya has no more connection to the Mediterranean or Atlantic Ocean (i.e. access to the sea) without going through Greek waters, that's an issue for Turkey. Greece already 'owns' about 50% of the Aegean, 12nm would extend that past 70%.
International law is the area of law least concerned with morality. Everyone disagrees but that's how it works because there's no such thing as a world court, world government, world police force. Just some semblance of it in our institutions that make a good effort but where political power is still key. Greece doesn't come close to Turkey's geopolitical power and has no explicit support from any major powers on this issue. That's why it hasn't changed from 6nm despite the law.
I would disagree by the way that they don't have valid claims. Perhaps under strict application of international law (which doesn't really exist) they'd lose. But they also make non-legal claims to equity and make claims about genuine concerns of the so called greek lake, which make quite a lot of sense.