When the government makes bitcoin illegal they will ofcourse claim it's to prevent child pornography, terrorism and drug selling. However the real reason will be tax evasion and the law would be successful in taking care of that real target.
The silly thing is that if they do make it illegal because some businesses won't report bitcoin profits to the IRS, that will just guarantee that all businesses (licensed or unlicensed) using bitcoin won't report profits.
Give the government a break. After all it is designed to benefit all! And without taxes -> no government. If you want anarchy, go to some country where they have civil war at the moment.
