There's nothing in what AB wrote about misbehavior. Unless (general) you consider it bad parental behavior to bring small children to a book signing, then his policy is not "rewarding bad behavior."
Apparently, though, a lot of people *do* consider it bad parental behavior. In some cases, it might be, especially if the kids are disruptive and the parents are doing nothing to control them. But I'm not willing to make the leap and say that a parent should never, ever, under any circumstances bring a child to a book signing. As a couple of PPs have suggested, the kid might even be a fan, too.
And he does say that this is something he does *sometimes*, and it's in the interest of allowing parents of wee ones to get the kids home to bed. So I'm guessing he doesn't do it at a 2 p.m. signing.
I've never been to an AB book signing, so I really have no idea how many people show up with little kids. But as a childless person, I would have no problem waiting a few more minutes while they went first. Especially if I knew that he was not going to bail before seeing everyone. I don't consider that discrimination against the childless -- which certainly does exist in the world, but Alton Brown isn't guilty of it IMO.