PKK is not a political party, it is a terrorist organization as recognized by many countries including US, EU,  and Russia. It does not declare itself on legal grounds, so it cannot be banned by definition. Moreover, by means of its organization and activities it has more of a military character rather than being political. Today, there is a political branch of PKK, named DEHAP, now transforming itself into DHT due to some internal conflicts between Kurdish separatists, which is a legal party and ideologically on the same line as PKK. A "political party" would not need a legal/political branch, would it?