I have the right not to sell poison to someone who I have reason to believe will use it to kill a third party. The idea of simply trusting the patron to be responsible makes sense when the patron is anonymous or a new contact. It’s generally good to assume good intentions in the absence of evidence, I think. If the government is not anonymous enough to get this treatment.

Governments have a long, long history of using "poison to kill a third party", to use your analogy.