HumblePie ---Uhh it is quite clear in the video that Brown shoved that clerk away from him as he was attempting to exit the store. If Brown did steal some cigar boxes, either some or all, then it is within the clerk's rights to attempt to get them back even if that requires use of force to do so. Brown using force back against the clerk's attempt to stop a robbery is considered strong arm robbery and is a felony.
If Brown paid for all the cigars, acted like a massive douche bag, and the clerk was just trying to be a douche back by blocking his way then Michael's shove is not an assault.
It all depends on whether he was actually stealing at the time or not. Which is hard to see in the video. Personally I think he was by the way he just grabbed a bunch extra and tried to leave. He may have paid for some, but I highly doubt he paid for them all.