If Americans want to make a law that bitcoin is currency - they are certainly allowed to make that law. Really.
If the IRS wants to make a law that bitcoin is 'property' - they can't. The IRS doesn't make laws. Ever. Today, the IRS said: "according to the laws we have today, we must treat bitcoin as property". If anyone doesn't like that, they can easily change the law. One or two cooperating congressmen - and this thing is up for full debate in congress. Congress doesn't care about bitcoin - they actually like it. We saw that already. It was funny - everyone here was amazed how favorable the lead congressman was. BTC went shooting through the roof in price. The US congress has only spoken favorably about bitcoin. You won't get much resistance to progress in the US. They like that shit over there. Why do you think so many cool new things come out of Silicon Valley? They love that shit.