It's all in the details and which side has the best P.R. standing at home. The P.L.O. was considered a terrorist organization by the United States and Israel up until the Madrid conference in 93 or 94.
The Karen Rebels are called rebels (I assume) because they are fighting a military junta. Along with this quote from wiki
The KNU is a democratic organization, and supports human rights and democracy in Burma.
I can't offer much information on Burma simply because it isn't seen as important over here in the U.S. The best I can do is offer assumption and poorly educated guesses.
Countries often label a group a terrorist organization when they use means of terrorism bombings, kidnapings, etc.
In World War 2 the French Partisans, the Francs-tireurs, French Forces of the Interior among others were considered terrorists by Nazi Germany. It's all about perspective.