Several of the most high-profile mass shooters in America, as well as many lesser known perpetrators, have been known marijuana users. The federal government understands this, which is why laws like this were put in place and the Justice Dept. has worked to disarm these dangerous individuals. As more states legalize marijuana, it's a foregone conclusion that potential shooters will get their hands on it, which is why law enforcement needs the tools to combat this growing threat.
The federal law that prosecutors were using against Harrison specifically prohibits only dangerous individuals from possessing firearms. As both the Oklahoma and New York cases involved people with violent histories, it seems the government has a violence problem, not a law-abiding gun owner and weed smoker problem. With hundreds of thousands of marijuana users in Oklahoma alone, no rational person would call all of them — or even a majority — dangerous people.