Cap,
For the same reason they smell asses? I have no idea, except I do imagine toilet water would be a goldmine for minerals (while everybody always cleans the bowl, nobody ever even looks in the holding tank…) According to the SPCA the reason is simply that you are a bad owner:
A dog or cat drinking from a toilet may seem amusing, but you’ll find yourself asking, “what’s wrong with the perfectly good bowl of water that you left out for your dog?”
When presented with a choice, it makes sense that dogs and other canines would prefer fresh water over stagnant water. In our homes, cool, fresh water is readily available in toilet bowls. To a dog’s sensibilities (though adorable, still quite different from ours), this is as attractive a choice as any, particularly if the behavior is reinforced by quenched thirst over the years.
To discourage toilet-drinking, water bowls should be washed and refreshed at least twice daily. It helps also to consider whether your dog likes what the water bowl is made from. To learn his preference, offer choices of stainless steel, ceramic and plastic. If offered his choice of bowls filled with fresh, cold water, chances are your dog will eschew the toilet-especially if it is kept covered for a while.
"In an honest Service there is thin Commons, low Wages, and hard labour; in this (Piracy), Plenty and Society, Pleasure and Ease, Liberty and Power; and who would not balance Creditor on this side when all the Hazard that is run for it, at worst, is only a sour Look or two at choaking. No, A merry Life and a short one, shall be my Motto."--Bartholomew Roberts