When establishing the EPA, Congress wrote a law saying "All dangerous chemicals will be banned. The EPA will define what 'dangerous chemicals' means." (Actually, the first iteration of the EPA just gave grants to local organizations to clean up pollution, but I'm ignoring that for the moment.)
'Is this chemical dangerous?' is a specific question with a specific scientific and medical answer. 'Is this a dangerous use of federal power?' is an inherently subjective question. So, that would be one reason why your example doesn't raise the same constitutional concerns as the FBI's request.
Second, being able to ban chemicals is a power that is much more limited than the power to make arbitrary companies do arbitrary things.
Nowhere. However, the Writs act does require that the writ be, "agreeable to the usages and principles of law."
Orenstien's point is spot on. There's no act of Congress that prevents the government from seeking a writ to provide execution drugs.
If we accept the government's argument that Congress must explicitly deny the request, and the argument that burden must be calculated purely based on financial cost, the only factor that would weigh against a writ for execution drugs is the discretionary factor of closeness.
'Is this chemical dangerous?' is a specific question with a specific scientific and medical answer. 'Is this a dangerous use of federal power?' is an inherently subjective question. So, that would be one reason why your example doesn't raise the same constitutional concerns as the FBI's request.
Second, being able to ban chemicals is a power that is much more limited than the power to make arbitrary companies do arbitrary things.