I'm not sure the IRS really needs to track all bitcoin transactions to find cheaters. Couldn't they just subpoena the records from exchanges to see who is selling for US dollars?
Not too broadly, they can't. With Coinbase, the IRS tried to go on a fishing expedition, and the courts sided with customers, allowing them to file John Doe motions against the IRS actions. In response, the IRS is reportedly drastically scaling back the probe of more than a million customer accounts.
Further, this happened on a licensed, US-based exchange. The IRS presumably has less leverage with offshore exchanges. Apparently, after last month, that's where the DOJ et al come in, to take down unlicensed exchanges and seize their money...
I would not be surprised if in the near future the IRS requires exchanges to report buy and sells, at least the ones in the US.
This is definitely going to be rolled out in the next year or two. Coinbase and Gemini will be issuing 1099-Bs to customers before you know it. The IRS doesn't want to miss out on this action any longer.