Because they demand recognition via acts of terrorism.
Like we can have a discussion about how closely linked Rojava are to them but the PKK itself is undeniably a terrorist organization that engages in terror tactics to pursue its aims of recognition as an independent state.
Just because we might agree with the IRA in principle doesn't change the fact that they did a fucking lot of car bombings, and the same principle applies here.
Terrorism = Acts of violence with the purpose of instilling fear. Or something along those lines.
Doesn't matter if you agree with them. I sympathise fully with the root purpose of the PKK and the ETA, and possibly also the IRA. I find that their means sometimes undermine their cause, and it makes me somewhat uncomfortable to be aligned with them politically.
It's possible for people to do terrible things in the name of a good cause.
And "terrorist group" doesn't mean "the baddies" - that's just how it's used in America. Tarantino's jewish gang in Inglorious Basterds would be a text book terrorist organization, but terrorising Nazis is generally considered an okay thing to do.
The best definition we have for terrorism is the threat or use of illegal violence to influence the people beyond the immediate area of the attack to achieve a political, religious, ideological or monetary goal.
It's not necessarily making a moral judgement about terrorism, just how it works. Al Qaeda is a terrorist organization because they used violence and threatened more with the goal of influencing US foreign policy on the basis of Fundamentalist Islamic theology. The IRA were also terrorists for their use and threats of violence with the political goal of unifying Ireland as one nation and taking it back from the British monarchy. Even the Founding Father's of the US could be considered terrorists by this definition. They destroyed property and killed British citizens in our bid for independence affecting the decisions of the monarchy and parliament an entire ocean away.
Wait, so if you overbake chicken until it's dry and I take a dump in the toilet's reservoir it doesn't make me the good guy? Well fuck.. maybe that's why we view teenagers as not having fully developed ideas sometimes