I personally think their cause is just. I am by no means a lore-master, but this is how my trail of thought has always worked when it comes to The Forsworn: They cannot possibly be labelled as a terrorist group when their cause is noble. They only wish to take back the land which is rightfully theirs. Empire's have risen and fallen on the basis of making others obey by their laws, and acknowledge them as their rulers. The Forsworn only wish to re-take The Reach, which was probably taken by Nords brutally.
In conclusion, The Forsworn do terrible things, but for a noble cause. Whereas The Thalmor and The Empire have done terrible things for a despicable cause.
Feel free to add your own opinions! =D