they picked on me because I'm a biker!

You are correct, in most cases there is no motive to assign. It's poor training, someone having a bad day, bad management or something like that. It has nothing to do with the person on the receiving end being black, white, asian, tall, short, male, female, gay, straight, biker, computer geek, single mom, single dad, senior citizen, christian, jewish, muslim, pagan, athiest or for that matter, doctor, lawyer or indian chief.

Does discrimination happen? Yes, I will absolutely say it does. I've experienced it as a woman when the mechanic tries to tell me the problem with my car is just "too complicated for me to understand" and I've experienced it as a disabled person, thought not to have the mental capacity to order my own dinner. I'm sure almost every member of one the groups above have felt it at some time or another.

The difference is most of us just write it off as someone being ignorant and not realizing that a woman can understand the workings of an internal combustion engine and someone in a wheelchair (not all the time) can actually read and interput a menu.

True malicious discrimination is hard to prove unless someone is blatant about it. In most cases, it's just ignorance.