What a stupid idea, I don't believe it'll get past senate but who knows. Regardless, this is capital control! Free convertibility is what holds U.S. Dollar together.
Sure, the taxes are only on non citizen residents. But that's precisely why it's stupid. Anyone can give their money to a citizen friend who can then send it abroad without taxes. Who's to say whether this a transfer payment/payment for goods and services versus someone willingly avoiding taxes? It's not like goods, it's just electronic entries.
Look at any country with capital controls, it is always a wide control. For example, Chinese residents can send $50k abroad in a year without any paperwork. This applies to all residents who hold Chinese Yuan, citizens and non-citizens alike.
Therefore, I have doubts on how effective it'll be.