Hello,
I don't think your example is quite the same. Backing a longshot is not like effectively just laying one horse to lose. But betting one player or one team in a head to head sporting contest is effectively no different to laying the other to lose.

If the TABs wish to be consistent they should abandon all betting on head to head sporting contests. Will they?

Of course the real issue is money. Everyone sees their massive income threatened by betting exchanges.

One argument I found quite interesting was the TAB saying how dependent Racing was on its income. I believe racing managed just fine before TABs ever started operating.