Right - but nothing is stopping them from making that law. Countries aren't simply going to adhere to a digital currencies protocol or allow it to avoid regulation because otherwise it goes against the "community spirit".

Here we are talking about the hypothetical situation of the U.S claiming complete jurisdiction of Ethereum. The U.S could make whatever laws it likes around the usage of Ethereum, legislate that all nodes have to adhere to these laws and job is done. The rest of the world can say "lolbye" and continue using it like nothing has happened, but in the U.S they would likely have to fork and create a new chain, and those who want to use it legally in the U.S would also have to use this new U.S-regulated chain. From there they can then implement processes to reverse transactions, introduce identification requirements etc.