No reason? This is the same guy who captured him by derailing a train and killing innocent people, and then tortured him in a government bunker. The same guy who did something with a giant robot or uh something, in that one issue of Diggle's AC that no one read.
I'd say Superman has a right to smack him around for saying "Let's quarantine you", and that's exactly how our new Superman should react. He's a man of action. He would totally smack Lex Luthor and take off. Screw the Justice League, and to hell with diplomacy, Lex Luthor is an evil man. That's my favorite part of the NEW 52 Superman. He doesn't just stand around giving speeches, he does what needs to be done. I respect a man of that fiber.