It's far more nuanced than that - the prior position never said that the activity was legally "ok".
It was an analysis based on the agency's prosecutorial practices in light of state law that is otherwise valid but for the federal drug law. The FBI wasn't involved in law enforcement work for the vast majority of possible implications of the state laws in question.
Every single prosecutor's office in the country, from the local city attorney, to the state DA, to the state BI, to the federal USAO, to the FBI has to make resource-based determinations of prosecutorial practice. It literally happens every day in this country.