U r right!!! You need a licence to own a metal detector here in Greece, and it needs time to get it (around a month)!
In addition, if you'd like to go out for hunting with this, you need an extra licence of exploring the area you like. This is more dificult to get it as they will send people from goverment to watch if you'll find anything so to take the taxis (over 50%)
On the other hand, here in Greece everything is forbiten and the same time everything is allowed (If you know what i mean).
The "problem" is because Greece is full of history as you already said. If you dig anywhere even by hands, you will find something for sure. And especially after the second world war, people were coming here with detectors and leaving with cases full of gold. Especially Germans and talians! Thats the why Greek laws are very protective about this matter.
I hope i helped a bit to understand what happens here about.
Yannis