The Muslim revulsion for dogs comes from Muhammad: "Ibn Mughaffal reported: The Messenger of Allah (may peace be upon him) ordered killing of the dogs, and then said: What about them, i. e. about other dogs? and then granted concession (to keep) the dog for hunting and the dog for (the security) of the herd, and said: When the dog licks the utensil, wash it seven times, and rub it with earth the eighth time." -- Sahih Muslim 551
And from the Shi'ite side of the divide, which pertains more specifically to this story, here is a contemporary voice -- Iraq's Ayatollah Sistani:
The following ten things are essentially najis [unclean]:
4. Dead body
8. Kafir [unbeliever -- i.e., non-Muslims]
9. Alcoholic liquors
10. The sweat of an animal who persistently eats najasat [unclean things].
Sharia Alert from the Islamic Republic of Iran: "The Latest Enemies of Iran: Dogs and Their Owners," by Azadeh Moaveni in Time Magazine, April 19:
For much of the past decade, the Iranian government has tolerated what it considers a particularly depraved and un-Islamic vice: the keeping of pet dogs.
During periodic crackdowns, police have confiscated dogs from their owners right off the street; and state media has lectured Iranians on the diseases spread by canines. The cleric Gholamreza Hassani, from the city of Urmia, has been satirized for his sermons railing against "short-legged" and "holdable" dogs. But as with the policing of many other practices (like imbibing alcoholic drinks) that are deemed impure by the mullahs but perfectly fine to many Iranians, the state has eventually relaxed and let dog lovers be....
Those days of tacit acceptance may soon be over, however. Lawmakers in Tehran have recently proposed a bill in parliament that would criminalize dog ownership, formally enshrining its punishment within the country's Islamic penal code. The bill warns that that in addition to posing public health hazards, the popularity of dog ownership "also poses a cultural problem, a blind imitation of the vulgar culture of the West." The proposed legislation for the first time outlines specific punishments for "the walking and keeping" of "impure and dangerous animals," a definition that could feasibly include cats but for the time being seems targeted at dogs. The law would see the offending animal confiscated, the leveling of a $100-to-$500 fine on the owner, but leaves the fate of confiscated dogs uncertain. "Considering the several thousand dogs [that are kept] in Tehran alone, the problem arises as to what is going to happen to these animals," Hooman Malekpour, a veterinarian in Tehran, said to the BBC's Persian service. If passed, the law would ultimately energize police and volunteer militias to enforce the ban systematically.
In past years, animal-rights activists in Iran have persuasively argued that sporadic campaigns against dog ownership are politically motivated and unlawful, since the prohibition surfaces in neither the country's civil laws nor its Islamic criminal codes. But if Iran's laws were silent for decades on the question of dogs, that is because the animals — in the capacity of pet — were as irrelevant to daily life as dinosaurs. Islam, by custom, considers dogs najes, or unclean, and for the past century cultural mores kept dog ownership down to minuscule numbers. In rural areas, dogs have traditionally aided shepherds and farmers, but as Iranians got urbanized in the past century, their dogs did not come along. In cities, aristocrats kept dogs for hunting and French-speaking dowagers kept lap dogs for company, but the vast majority of traditional Iranians, following the advice of the clergy, were leery of dogs and considered them best avoided....