You don't need a US-based exchange for BTC, per se, as other currencies are traded on the open market and the brokers simply relay quotes to their customers via feeds they receive rom the banks. If the banks saw the potential to make money off of trading BTC, they'd make it happen.
You don't need a US based exchange, but changing bitcoin into a foreign currency and then turning that foreign currency into dollars would probably incur enough in transaction fees to be unprofitable, especially if you're changing large amounts of currency on a regular basis.
If you read into what Forex is you'd discover that what you're speculating about actually happens, for pure profit, many many times over on a daily basis.
