Maryland is one of the 23 states (and the District of Columbia) that have most recently relaxed their laws regarding the possession of marijuana, and although Pennsylvania still holds to the old statutes, it is now decriminalized in Philadelphia at least. In other words, you can no longer be arrested for having a certain amount of marijuana i.e. less than 10 grams in Maryland and less than an ounce in Philadelphia. Instead, you are levied a fine i.e. $100 in Maryland and $25 in Philadelphia.
However, the issue of marijuana legalization is still a hot topic among legislators, many of whom believe that decriminalization of the substance creates more issues than it lays to rest. For example, it would require law enforcers to make judgment calls about whether the amount of marijuana they find warrants an arrest or not. It also brings up the specter of racial disparities in doing searches for suspected illegal possession of marijuana.
State law enforcement has a reason to be nervous. Making an unwarranted arrest can cost the state a bundle in lawsuits, especially there seems to be an overrepresentation of a racial group in the books. It may just be safer (and less expensive) to simply look the other way.