It could be argued that just as she had to convince herself that the pitcher could not be used as a weapon, you could convince yourself that the collar as not a weapon. It does not intrinsically cause physical damage. Once it is applied and she became Renna's Sul'dam, it was a tool. Even according to Seanchan ideology Damane are trained to serve, not "punished". The act of hanging Renna up was physical and not in any way affected by the collar.
Renna could have tried to exert control through the collar, but it is conceivable that she was in shock at being affected how she was. She KNEW that she couldn't channel. She KNEW the collar couldn't control her. Her worldview exploded in that moment. Then she panicked and let Egwene go in an effort to save herself. Big mistake, HUGE.