As I understand it (usual disclaimer, IANAL etc.), the law under which he is being prosecuted is a law designed to protect people from wiretapping. Most states have a one party rule, which basically means that a third party can't record a private conversation between two people, but either of the participants in the conversation can (the "one-party" bit referring to how many parties to the conversation have to be aware of the recording before it is legal). Maryland, on the other hand, has a two party rule, which means that nobody is allowed to record any private conversation unless all participants are aware that the conversation is being recorded. So, the motorcyclist was supposed to turn off the camera as soon as a private conversation began. The problem here is that the guy who recorded the conversation either a) did not know about this law, or b) did not believe it applied in this instance [in the interests of even-handedness, I should probably add "or c) knew about the law, knew it applied, and violated it anyway"]. The real issue is that the guy, and most of the commenters here, by the looks of things, do not believe a traffic stop in a public place constitutes a private conversation between the individual and the police officer.
This is where things get a little tricky, because (at least in the UK), if you are stopped by the police, you are legally required to give your name and address, which is private information. So we have a situation where two individuals are in a situation where one party is unable to legally refuse to give private information in a public forum - so why is this conversation not afforded any sort of privacy rights? On the other hand, it's quite clear that, as others have said, if, for example, you are filming a school play by your child, and the couple next to you have a private conversation, they have no reasonable expectation of privacy.
My personal opinion is that technically, legally, Maryland probably have the slight upper hand, however, any jury that finds for them should be shot. I also believe though, that this is a use of the law outside of the spirit in which it was written, so there is a case for the defendant there. I shall be following this with interest.