At least insofar as the private sector is concerned, there are no laws whatsoever regarding internal job postings. Or even external ones, barring a couple that relate only to employment visas for non-US citizens. In the absence of a legally binding and enforceable contract or CBA that expressly says otherwise, an employer can do anything they want with regards to job postings as long as they do not violate discrimination laws. They can even violate their own policies. And nothing whatsoever in law says they have to hire the person that you consider to be the most qualified.