Aside from the obvious ("because it only encourages them"), the U.S. Food And Drug Administration offers another, more worrying reason:
A number of Americans who placed orders for specific drug products over the Internet (Ambien, Xanax, Lexapro, and Ativan), instead received a product that ... can cause muscle stiffness and spasms, agitation, and sedation ... Preliminary analysis indicates they contain haloperidol, the active ingredient in a prescription drug used primarily to treat schizophrenia.
Ouch. No surprise here: spammers are Bad People. Lest we forget, spam isn't merely an productivity-sucking irritant.
I'm indebted to the FDA for providing the following photos and captions. More at the FDA site.
Back photo of yellow Haloperidol with “H 2” imprinted on the tablet.
Front photo of yellow Haloperidol with “Janssen” imprinted on the tablet
Plastic bag containing yellow Haloperidol tablets as received by consumers
Mailing envelope in which tablets were shipped to consumers
Mailing envelope in which tablets were shipped; yellow tablets, and the clear plastic bag in which they were contained. The ruler was placed for size comparison purposes
Mailing envelope in which the tablets were shipped to consumers. The ruler was placed for size comparison purposes.
White Haloperidol in blister pack
Yellow Haloperidol in blister pack