I am not an expert, but I remember when participating in aircraft evacuation training that I was surprised by how much the flight attendants have to do during an emergency descent or depressurization and emergency landing (including the constant shouting of emergency instructions at passengers).

It definitely seems that flight attendants have a high workload in those situations, so hopefully they don't have to worry about dismantling seatbelt barriers as well. I suppose it's possible that they would have the time and the presence of mind to remove the seatbelt barrier while in the emergency descent and before access to the emergency exits is needed, but I am guessing they would prefer to spend their time focusing on their primary emergency duties in that situation.