If you're an "at will" employee, without an employment contract or a union membership, you can be fired without notice at any time for any reason or no reason at all, and you're able to quit without notice and for any reason or no reason. So they can fire you, or not hire you for future work, for not buying their product. If buying the company's products isn't too onerous a requirement, then maybe keeping the job is worth it.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.