I still remember that article about recreation that had a picture of people CLEANING THE KINGDOM HALL PROPERTY as a form of recreation. If they can present a picture like that, I think pretty much everything non-Jehovah-related is to be cracked down upon. It would be impossible to work full-time, engage meaningfully in field service, study for and attend all the meetings, do personal study, prepare for and do family study if you have a family, and still have time or energy for anything other than sleep.
Also, didn't gladiators actually, you know, fight to the death? I'm pretty sure that's not the case in football. While injuries do happen, I don't see how that would be entertaining. What makes it exciting is the movement of the ball; I'm not eager to see somebody get slammed into the turf/grass/whatever. To me, it's no longer fun when someone gets hurt.
Never made a habit of watching boxing, though I've seen a few fights (none with JWs, though). Used to watch wrestling, too. But eventually I realized I wasn't comfortable watching it when mom was around, so I gave it up. Years later it really dawned on me just how violent it looked, even if it is 'fake'. (I say 'fake' because it's not like real injuries don't happen and those wrestlers rarely make it beyond their 50s, and I'm sure there's a very specific reason for that...)
Anyway, I suppose the Bible's sex and violence will have to suffice...