As I recall, the DEA is a Federal agency; enforcing Federal laws. How does a state making pot legal help if nabbed by a Fed?
First off, the feds hardly ever make any simple possession arrests. They only make a tiny percentage of the total number of possession arrests made in the country every year. Almost all marijuana possession arrests are done under state laws.
Moreover, Massachusetts did not legalize pot. Understand that there is a difference between legalization and decriminalization. Decriminalization just means that they remove the threat of jail and in most instances where states have decriminalized it is no longer considered a criminal and doesn't leave people with a criminal record. It's still illegal, but when police nab someone for simple possession of a small amount for personal use it is handled more like a traffic ticket. They don't have to arrest the guy and take him to the jail and book him in like they would in most states where it hasn't been decriminalized. It has already been decriminalized in several states, and most that decriminalized did it in the 1970s. Decriminalization has not been a problem for the states that decriminalized. Per capita use of marijuana is about the same in states that have decriminalized and those that have not. This new law in Massachusetts is no big deal.