To maybe extend good Sankha's answer for good use:
When the monks feed them they end up staying there.
That why there are restriction in sharing food even toward lay people, to avoid material dependencies and inproper use of what's given toward higher (critic) and improper relations. So a monk wouldn't give food further personaly in most cases to not monks. It's improper to care about animals when living on alms dedicated toward Sublime. Yet of course it might be nevertheless that dogs dwell near avalible food, even if no relation is build up. From experiances, if not giving into any socializing, even if sometime coming, dogs wouldn't stay for the rest of alms in the forest longer as 24h. Some, how ever, would pleased participate when people take short retreats, even keep 8 precepts that time, at the same time stay joyful but respectful toward the recluse and the dwelling, not feeling "home".
In short: It's total improper, for those gone forth, to live in improper assosiation with non-fellows, yet not to speak of sharing living with dogs or hold them even as "own". At least there is a reason why animals are not given to go forth under the Gems.
Being easy suspected for improper conducts, socialization of any kind with animals are avoided, given that such is even measure for many kinds of human groups.
On the matter of association with animals more here:
