IANAL, but my understanding is that this isn't true, and the whole subfield of tort law [1] is about the rights and obligations that firms have even in the absence of a contract. If you steal trade secrets from a competitor, even if you have no contract or business relationship with them, they can still sue you for a very large amount of money and possibly get an injunction against using that stolen information. Similarly for cases of patent/copyright infringement, libel, slander, price-fixing, antitrust, etc.
I suspect that a large class of these "business ethics" violations are actually torts, but the problem with enforcing them is that to do so you a.) need to know what your rights are b.) need to detect violations of them and c.) need to bring a lawsuit against the offender. I remember that on Google's invention disclosure form, one of the questions was "How easy would it be to detect if a competitor was using this invention in their product?", and if the answer was "We'd never know", we wouldn't patent the idea and would keep it as a trade secret instead, on the assumption that if a competitor could use the invention illegally without us knowing about it, they would, and so it was better to deny them knowledge of the invention entirely than to publish and try to enforce unenforceable rights.
When the aggrieved party is a consumer or worker, it's triply difficult, as most consumers a.) aren't aware of their rights b.) have no information about what's going on inside big corporations and c.) don't have money to sue even if they know. Many of the ethical problems in U.S. business today stem from the legal system not being able to scale up to 300M often poorly-informed citizens.
[1] https://en.wikipedia.org/wiki/Tort