Without capital gains, if Bitcoin had ~0% inflation and the US got back up to 5%-10% inflation, then Bitcoin would become a better store of value.
However, with capital gains, the US would have to have significant inflation, around 20% or more, before Bitcoin would really be competitive. (I'm assuming there are significant transaction costs related to the low acceptance of Bitcoin)
In this manner, the US can (1) gain some additional tax revenue, and (2) prevent Bitcoin from becoming a competitor to the national currency, which every nation wants complete control over.
Just to be clear, I'm not agains taxes, because not paying taxes is like swatting an apartment, and generally taxes make sense to be paid, although most places also have several unfair tax codes, so I understand the common perception of taxes as "highway robbery".
Do the IRS and say Turkey's tax office tracks these as well? Assuming that you don't try to deposit the exchanged for bills in a bank.
Couldn't someone offer Bitcoin exchange in existing currency shops or the same service via Bitcoin ATMs? How would the IRS and its equivalents track that without requiring identification and registration before use? I suppose a small amount would be allowed for the tourist aspect but then you could use a group of friends or employees to wash small amounts. If you require ID for that, including tourists, then you can track it.