The way I see it, if any government does decide to regulate and tax Bitcoins, keeping an eye on exchanges is the way to do it. There would be no way to monitor every single person and every single one of their transactions. They would have to depend on the individual's honesty and integrity to file the proper taxes, which would honestly be not much better than not taxing at all. The way things are now, you could move massive amounts of Bitcoins and not have any use for them because very few stores accept them. In the end, you're going to have to convert them to fiat, and the most efficient way of doing that is through exchanges. This way, if you impose taxes on exchanges, then you've effectively taxed majority of a person's transactions.
I would honestly rather live in a world where Bitcoins and exchanges aren't regulated, but even regulations have positives, especially when it comes to getting Bitcoins into the mainstream. Right now we'll just have to wait and see how things go.
There are many ways to completely regulate Bitcoin for government and if someone convert Bitcoin to fiat and they can easily tax them. The major way will be to close down every exchange of the country and that would be easy since exchange needs bank and open a government regulated exchange where they can monitor the inflow and outflow of money and keep a check. This is what I think China is going to do by launching their own cryptocurrency.
Bitcoin can stay anonymous and out of government control till they are on internet as crypto. Once they are converted to fiat, government have many ways to regulate them. The things is bitcoin still isn't a bigger a phenomenon that countries need to fear or do something major about or against it.Yeah, this is my point in a nutshell. That's why I was saying that governments that are looking to tax cryptocurrencies are most definitely going to keep an eye on exchanges. Bitcoins aren't anonymous at that stage anymore since most exchanges ask for a bank account to deposit fiat to. It would also serve as a way to protect their own currency, as they will be able to monitor the flow of money. It definitely won't be as easy for money launderers to do business.
Regarding Bitcoin as a phenomenon though, it's not big enough now, but we're already seeing more and more countries taking a stance. This is a sign that it's growing, and I only hope they don't take steps to stifle that growth. Blockchain technology is the future, and cryptocurrencies are just one of the many applications. They should at least keep it around and see how it evolves.