Why can't I practice full contact kickboxing if I have had my jaw broken?

I had my jaw broken in April 04 and had a metal plate put in. I have since joined a kickboxing club but they have advised me that I cannot participate in full contact kickboxing because of it. I was only advised by the hospital that I wasn't allowed to box for a year after the incident. Why are they telling me something different and can I sign some kind of disclaimer to say I don't mind full contact kickboxing? I can't find any website which helps. Thanks