Battle forces are a collection of loose sprues for those kits in a bigger box. If they can't even stock the little boxes to meet demand what chance is there for them to have the sprues for the battle force?
Well I had that thought, but chances are that these things are boxed up weeks before the release date starts, so the unanticipated demand wouldn't really effect things. Unless there's some reason that GW knew they couldn't cast up as much as normal for the Tau release... Whether that's supply issues down the line, or maybe they have something else on the go that's taking up production space/time </conspiracy theory butter>