That’s interesting. So when you cash out on a legal sports betting app they do not withhold anything from your earnings?
I can only say how it works in horse racing. I use an online service for all my bets. I deposit money and bet off what I deposit. They do front you the money in the respect that you can start betting right away. They don't make you wait for the transaction to clear.
I have made about 800 bets this year and have cashed about 48. No tax has been taken out of any bet. I had a Pick 4 that paid $1,800 that not only had no taxes taken out but wasn't even reported to the IRS because it didn't hit the 300-1 threshold. When you take money out of your account, they don't take any money out for taxes ever. If you are lucky enough to hit a big one, they take the tax out first then deposit the balance in your account.