Government logic...

Government logic...  Megan McArdle notes some government information that is internally inconsistent.  Some time ago, Washington D.C. implemented a tax on plastic bags to discourage their use.  Recently they declared success, with plastic bag use down to less than half of its former levels.  But, Megan notes, the tax revenue from the plastic bag tax is going up.  How can that be?  If the number of bags being used has been cut in half, shouldn't the tax receipts be half as well?  Megan digs in...