Of course it's someone else's problem. That's why you're paying someone for the service. We all do that. As you partly recognize, if you legally contract with a company for a service (waste disposal for instance) you don't expect to be liable morally or legally for mis-conduct or failures by the service provider. Why should an exported service be an exception?
Doesn't this assume all service providers are morally and legally good? I would restate the principle as, "you're morally and legally liable for the misdeeds of a service provider with which you've contracted in accordance with the extent to which you can reasonably expected to be aware of their moral and legal practices." But if you assume all companies are legal (else they wouldn't be companies) and that legality implies morality (an absurd belief), then your justification would be spot on. It's a gray area, in my uninformed opinion, and a key question whether Western states could reasonably expected to accurately assess what China would do with their trash
Well knowledge is also a component to it and morality and legality.
To give a black comedy example if you have a friend who is a real fan of Tony Soprano and you ask a look alike to hold a surprise party for you and he is a hitman with a front company instead of an impressionist who takes it as a euphemism to murder them their death - even if you were a negligent idiot is vastly different than you straight up hiring a hitman to kill them.
