Listen, you don't think the US would have allowed Bc to exist if they weren't having a plan to tax it. Bitcoin was accepted by US authorities for 2 reasons IMO : 1) they saw that the US is actually the country benefiting the most from bitcoin in terms of business creations, and they foresee potential technological advances arising from this fad, 2) just like weed, they see bitcoin as a new and very much welcome source of tax revenues, from bitcoin companies and of course from the so-called investors.
So they went along with this madness figuring the country may profit from it as well. It's like all bubbles, the government is happy to play along and get taxes until everything collapses and all of a sudden, tax receipts are way down and they are in trouble for not preparing for the disaster.