Careful about the "warrantless" here. It's true, it's a warrantless search, and don't think I'm saying that's an illegitimate concern. However:
The police already have broad authority to search you and your immediate person incident to an arrest. If this is news to you, you really want to read up on it. This is one of the big things that makes a formal arrest different from mere detention, and one of the reasons the ACLU guides and things like that instruct you to clearly ask if you're "under arrest".
All the DoJ is saying here is that a cellphone is, like your pockets and your bags, a legitimate target of a search incident to arrest. There's apparently a circuit split on the issue. There are other things that aren't legit incident to arrest; for instance, certain kinds of car searches aren't lawful solely because of arrest (trunks can be searched if cars are impounded, but only for "inventory", and can't be searched IIRC if the car isn't impounded; similarly, being arrested outside won't give the police the automatic authority to search your house).
The police already have broad authority to search you and your immediate person incident to an arrest. If this is news to you, you really want to read up on it. This is one of the big things that makes a formal arrest different from mere detention, and one of the reasons the ACLU guides and things like that instruct you to clearly ask if you're "under arrest".
All the DoJ is saying here is that a cellphone is, like your pockets and your bags, a legitimate target of a search incident to arrest. There's apparently a circuit split on the issue. There are other things that aren't legit incident to arrest; for instance, certain kinds of car searches aren't lawful solely because of arrest (trunks can be searched if cars are impounded, but only for "inventory", and can't be searched IIRC if the car isn't impounded; similarly, being arrested outside won't give the police the automatic authority to search your house).