Hernandez was involved in a MURDER, which is why he was cut immediately. DWI is not exactly the same level. You'd look pretty retarded cutting him immediately if all he ends up with is having to do his (already sentenced) 30 days immediately, or more community service, whatever.
It's not just the DUI; he was found parked on a highway, straddling between two lanes. He was also on probation for a previous incident last year, where he PUNCHED A COP.
It's the middle of the offseason right now, so 30 days is no big deal. But the main issue at stake is if this a player/person who is prone to issues with law enforcement, and whether or not these issues could potentially lead to the player not being available for a key game during the season because they're spending the night in jail in the future.