Not justifying the ban, but they did spray the engines (and by the looks of the video they sprayed *into* the engines) putting the operability and safety of two large aircraft at risk. Those engines will require complete strip-down and rebuild before they can be operated safely.
Was it an act of terror? No. Was it very, very ill-considered? Yes. Could it have put lives at risk? Directly, no, not if inspection protocols are followed. Should they group be banned? No.