Because countries have customs enforcement that (in theory) check imported goods for things that are illegal. Of course things can slip through, and often do, but on the whole you won’t have a mass market item that is being sold to millions of people slipping through
Customs isn’t just about criminally illegal things like drugs, they also ensure goods meet laws like your asbestos example, or that they don’t infringe on trademarks (customs will often look for fake designer goods like handbags), or that they meet safety standards (think cars), etc.
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