You're right about states not being prohibited from enacting laws governing interstate commerce, I think the clearer argument is one of jurisdiction. States ha

You're right about states not being prohibited from enacting laws governing interstate commerce, I think the clearer argument is one of jurisdiction. States have no right to compel a company that is not doing business within their borders to collect sales tax. States can and do collect tax on interstate purchases, they just do it by compelling the individual taxpayer to report and pay tax on those transactions. The problem is, without any way of auditing those transactions, there's basically no way to enforce this rule.