I think people usually have a half-good reason for doing what they do. At half-time during one match last season, some bloke handed me his (untouched) pint saying he'd got a bit of an emergency and had to dash home. Certainly had no problem with him leaving early!
Sometimes it needs to be pointed out. And don't you think it's hypocritical to criticise people for arriving late when you annoy others yourself by leaving early?!
Such a boring thread. You'll always have people arriving late, leaving for half-time early, getting back to their seats after half-time late, leaving early.
The half-time thing is undoubtedly due to how long it takes to get served at the interval. I'm sure virtually no-one deliberately arrives...