Re: Why do browsers allows JS from other domains to run
"I've never understood i) why a site would trust other sites to host code for them"
Because they're cheap and lazy and don't care.
"and ii) why browsers allow one site to run scripts from another."
Because if they did they'd get a reputation for breaking all the sites that were cheap, lazy and didn't care and everybody and their Facebook friends would dump them in favour of browsers who didn't care either.