In the cake example, a baker absolutely could attempt to negotiate for chocolate. They might not "win" the negotiation, but they have the option.
Edit - in the taxi driver (traditional taxi in US), I'd argue that most of them should be employees too. Sometimes they are employed directly by "Acme Cab" or whatever. But, in the case of the local airport taxi company (Washington Flyer), the company has a monopoly on airport transport, but drivers are independent contractors - this always struck me as a very lopsided and questionable arrangement. If a driver wants to pick up at Dulles, they have to contract for Wash Flyer at fixed rates, renting a company car, etc.
And I'll freely admit this all can be a bit ambiguous. And that I'm not a fan of the "gig economy" in general - I'm convinced corporations are using it to reduce wages and benefits and effectively foist the cost of those lower wages/benefits on society as a whole.