Couple of things...
1. If they had a warrant for his arrest, they probably didn't catch him in the act and arrest him on the spot. They wouldn't need a warrant for that. It might be the result of an undercover police operation in which he (allegedly) sold marijuana to an informant or a police officer.
2. It is likely that he has been indicted already. That's usually how these things work. The police do an undercover operation, get the goods on as many people as they can without blowing their cover, present a bunch of cases to the grand jury, indict the suspects, and then issue warrants for their arrest.
Just an educated observation on my part.