The same way they distribute anthrax now: send out a lot of "packages", but only load up a small percentage with the real thing. In this thread, there's a report of an arab buying a large quantity of candy, then returning it. The police "checked" it, and then it was put back on the shelf. Does anyone believe that every piece of candy in that lot was tested for every possible contaminant?
I am advising everyone I know to not by any halloween candy, and to not allow their children to accept any -- even "from people we know", since it can be tainted at the retail level.