Actually, it's in the best interests of both the rebels and the terrorists for the terrorists to watch from the sidelines.
The rebels, obviously, don't want to be conflated with terrorists, because that gets munitions with "Made in USA" stamped on them lobbed in their direction. It would also give the existing governments increased international legitimacy.
But it's also in the terrorists' best interest to stay on the sidelines now. Why? Because then they can cozy up to whichever side wins (or operating with complete autonomy in a failed state like Somalia). Terrorists have no particular interest in governing, but they do have a strong interest in having a sympathetic government to provide cover for running training camps and other facilities.
So who does benefit from apparent terrorism? The current governments, but only if they can successfully blame it on terrorists. A successfully faked terrorist attack will give the existing government increased international legitimacy and undercut any international support for the rebels.