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.