As someone said in a previous thread today, that's why it's called a living allowance, not an income. So they don't declare it as income to the IRS and pay tax, and don't declare it to the church and pay tithing. It is to differentiate between the rank and file (who have real jobs) and GA's who get an "allowance." So no salaried GA's pays tithing.
It's all legal in America but of course its is all a game of semantics that isn't fair when held up to what's honest.
I've been told (not sure about the reliability of the source) that GAs don't pay tithing on their "stipend" or anything else they get from the church, but most (but not all) do pay tithing on any other income. Some still own businesses, or hold stock, or have other investments that bring in money apart from what the church gives 'em.
Most GAs probably have been through the Second Annointing ceremony. As long as they don't kill someone or aposticise, they're guaranteed to go to the highest level of the celestial kingdom, to become gods of other worlds, and to create an eternal line of offspring. At least that is what the theology says. With that kind of guarantee, you would no longer need to pay tithing. You could even take up smoking and drinking if you wanted to.