Reggae in common?  Potheads in the audience?

It's probably what he or she said. Matisyahu's entire act and persona has been co-opted by the music industry.

311 are the headliners. They (along with the record company) select the opening act. 311 likes Matisyahu and can sell enough venues out so that they get their way. End of story. Oh, yes, and the reggae. And the herb (I've seen 311 close a show rith "Who's Got The Herb").

Maybe it's just to show that "311 is Down With the Unity".

"Can anyone tell me more about 311 & why they are appearing with Matisyahu this summer?"

I think it's really funny that you thought Matisyahu was bringing 311 on tour, not the other way around! I mean, 311 has been routinely selling out arenas since around 1993, whearas Matisyahu has what one might charitably call a "cult" following. A more uncharitable description would be that he's a novelty act.

Don't get me wrong, Matisyahu's pretty good, but I can only take so much of him.

It is something called a Unity Tour