The reason, as frequently related to me by friends in Amsterdam, that the Dutch hate drug tourists, via the BBC:
The Dutch government is banning the sale of all magic mushrooms after a series of high-profile incidents involving tourists who had taken them.
Luckily, stupid American tourists aren't the impetus for the ban:
Calls for a re-evaluation of the drug grew after a 17-year-old French girl jumped from a building after eating magic mushrooms during a school trip to Amsterdam in March.
Other incidents involving the drug have included an Icelandic tourist jumping from a balcony and breaking both legs and a Danish tourist driving his car wildly through a camping ground, narrowly missing sleeping campers.
This is reminiscent of the wonderfully offensive Onion headline, "Fun Toy Banned Because Of Three Stupid Dead Kids."