~snip~
the limit should be unlimited. if someone has already pay taxes on money they put into bitcoin, why should they have to pay taxes on it again? eventually the government wins and gets everything.
They are different taxes though. You never pay tax twice.
When you receive money from work, you pay income taxes.
If you buy Bitcoin with that money and then immediately sell at the same price (let's ignore exchange fees for now) you don't have to pay any more taxes.
You only pay taxes when you sell Bitcoin (or any other asset) at a higher value than what you paid for it. That's capital gains tax. You're paying on the extra money that you have.
You only pay on the extra money, not the initial one that's already taxed.