Because the vesting clause provides the power to tax in order to sustain the government and provide for general welfare services, military, etc. punishments are handed down via due process of law and taxation is not one of them.
so a 90% tax on all single white males with brown hair between the ages of 25 and 45 would pass your constitutional muster?
If prohibitive taxes were allowed, then all congress would have to do to prohibit an activity, be it a natural right or civil right, is tax it so much that the cost prevents its exercise. That is why a tax cannot be prohibitive.
edited to change:
Actually, read McCulloch v. Maryland (1819)
http://en.wikipedia.org/wiki/McCulloch_v._Maryland