Lost revenue.
Who is going to buy a match ticket for an evening game (there must be some sold) if there are 2000 or so unused kid's tickets that will admit them for free? Everyone would know someone who could match them up with a spare. I have two for every midweek evening game, for example.
Well, for starters, 66.6% of my three tickets are kids. Neither were there on Tuesday, and there is scarcely a more committed Albion fan to be found, than my oldest. It was simply not POSSIBLE to get him there after college (in Winchester) and nor was it acceptable to get him home at midnight...