I agree that it's just plain rude to bring an animal, really any animal, to someone's house without first clearing it with them. And this goes double for a stray agressive dog. At the very minimum, they should have called the hosts and cleared it with them before ever even trying to corral the animal in the first place.
Those of you who suggest keeping it outside, well, sorry, that presents a whole other set of problems. For example, I don't have a fenced in yard. Should the animal be tied up in the back yard to keep it from escaping? And regardless of the existence of fencing, what if it barks all night? What if it digs in the yard and causes damage? What if it somehow gets loose anyway?
Honestly, I do have a lot of sympathy for stray animals and would certainly rather see them get back to their owners or find a home if possible rather than go to the pound. But I'm no more equipped to handle that animal than I am equipped to help the homeless person in the above example. Does that make me a bad person? I really don't think so. There are plenty of people who would step up to help that animal and I applaud them. But only if they are doing it in their own realm of responsibility. The minute you wrangle the help of someone else who didn't also volunteer for the task, you've lost my admiration.