They do tax crypto trades as usual trades (non-crypto), as of 2018.. sort of. Non-crypto trades involve USD for U.S citizen (typically). Meaning when you sell something you are taxed on it. Now whenever you make a crypto-crypto trade you owe taxes on gains. So even trading in and out of bitcoin you owe.

Before 2018 the allure of tether was that you were only taxed on crypto fiat trades.