i feel that before you start anytype of discussion/debate, you should first ask the question if they know why it is legal. Then you should tell them all your facts, the whole time, continually asking them why they think it could be illegal, maybe you can jog their memory to thinking some crazy excuse as to why they think it should be, each time chopping down their answer to there is no reason for it to be illegal.

When your done, they will still believe that it should be illegal, but what else is new, atleast you got someone started as to how it should be thought of, maybe they will do some research of their own and decide it should be legal.