Earlier quoted context omitted.
When you engage in commerce you're subject to regulation, per the Constitution. Which means giving up (unstated) rights that you otherwise would have as a citizen. Discrimination for example. Social contract theory ;)
Only interstate commerce is regulated in the Constitution.
Article I, Section 8, Clause 3: [The Congress shall have Power] To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;
Several high court rulings have concluded that it includes intrastate commerce. It's wide reaching & has historically been the main tool for combating discrimination.