The Maryland law is different from the law in other states. The article text reads, "A dozen states require all parties to consent before a recording is made if there is a 'reasonable expectation of privacy."" First of all, it's very dubious that a state law enforcement officer would consider himself to have an expectation of privacy while interacting with citizens in public places. Fortunately, most states have a more reasonable body of law (like mine) and allow any party to an interaction to record the interaction without permission of the other parties (which is also the rule of federal law on taping interstate telephone conversations, absent a more restrictive rule of state law). It's outrageous that members of the public can be prosecuted for recording the public activities of law enforcement officers. It is also unusual for such a prosecution to be possible.
After edit: Another HN thread a while back recommended the book Arrest-Proof Yourself: An Ex-Cop Reveals How Easy It Is for Anyone to Get Arrested, How Even a Single Arrest Could Ruin Your Life, and What to Do If the Police Get in Your Face
http://www.amazon.com/Arrest-Proof-Yourself-Ex-Cop-Reveals-A...
which I greatly enjoyed reading and which my son is reading this week. There is a lot of good information there about how and why to stay out of trouble with the police.