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 ;)

It makes sense to me that businesses should be regulated and should not have the same rights as a citizen (Citizens United?)

One thing I wonder, though, is: what if the owner or sole proprietor "quits" their business? Are they still criminally/civilly liable after they've "given up" their additional role? And, if something like that'd actually fly, would they ever be able to return to their [line of] business, or would they have to quit forever?

Reminds me of Lavabit, that secure email service whose owner shut down rather than complying with an order to give the government information on a client.

Only interstate commerce is regulated in the Constitution.

Not correct.

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.

In some ways, and in some states, you do. You also take on a greater responsibility to the public, to whom you're offering products and services, that you would not shoulder as an individual.