Joe Random web developer doesn’t necessarily care whether the browser he personally develops against is disrespectful of someone else’s client-side resources. He just wants to make his website with as little busywork as possible, and testing every website feature for cross-browser compatibility is busywork. If he can avoid the trouble by telling visitors to switch browsers, he will be happy to do so.

This is not hypothetical: developers have been putting up “this site works best (or only works) in X browser” warnings for decades by now.