why does the police keep telling me my card is a fake or not good in san bernardino co. and confiscate my marijuana

i live in grand terrace, california and every time my friends get pulled over for bogus reasons they tell the cop that they have marijuana and have a medical card to carry it. well the police here say our cards are fake or that they are no good in our county only in L.A. county. Then they either take our pot or make us stomp it out. the more we try to talk to them and tell them that our cards are legal they get upset and tell us good luck proving it in court. that would be easy if they actually got tickets to appear in court. they just say next time we will be given tickets. i dont know, im confused and dont know where to turn for advice. the police dept. sure wont give you any answers. can anybody tell me how to prove my card is good before the police stomp it, or take it? how can i handle the situation next time the police harrass us? because harrassing us is all they do. our police here are corrupted, seriously. alot of them have been suspended for harrassment, but when they return, they pay you back for reporting them. trust me, ive had a cop tell me that to my face as i was getting a ticket for a modified exhaust. i dont even have a modified exhaust. thats what im dealing with. can anybody help me out?