If he dressed himself before trying to enter my store, I’d probably let him in. However I’d keep watching him; and on his first attempt to take off his clothes, whip his dong or throw poo, I’d throw him out.

Note however that there’s an important difference between both situations: defederation affects a whole group of people. We could claim that some of them were whipping their dongs, were throwing poo, but we can’t automatically point at all of them and say “you’re all at fault”; that would be incorrect and immoral.

Note also that I’d likely see this in a different way, if the defederating instance was beehaw instead of lemmy.world. Due to the purpose of each instance (generic and free-for-all vs. safe space), and overall impact on the lemmy side of the Fediverse.