Discipline in the US is governed less by a single statute than by the interplay of at-will employment, federal labor and anti-discrimination law, and state-specific rules. The baseline is that roughly seventy percent of American workers are employed at will, meaning the employer can discipline or terminate for any lawful reason or no reason at all. Progressive discipline is therefore a best practice, not a legal mandate. So why write anyone up? Because the at-will rule is riddled with exceptions, and a documented, consistent process is your evidence that you acted for legitimate business reasons rather than discriminatory or retaliatory ones.
The National Labor Relations Act (NLRA) shapes how disciplinary meetings must be conducted. Under the Supreme Court's Weingarten decision, union-represented employees have the right, upon request, to a representative during any investigatory interview they reasonably believe could lead to discipline. If you deny that request and continue the interview, any resulting discipline can be struck down as an unfair labor practice. The right currently applies to union workforces, though the NLRB General Counsel has pushed to extend it to all employees, so prudent employers treat the courtesy as universal. The National Labor Relations Board's official explanation of Weingarten rights is the authoritative reference here.
Anti-discrimination statutes set the outer boundary on why and how you discipline. Title VII, the ADA, and the ADEA prohibit using a write-up as cover for bias, and they make consistency essential: disciplining one employee for tardiness while ignoring another invites a disparate-treatment claim. The FLSA matters too, because docking an exempt employee's pay as a disciplinary measure can destroy the salary basis that supports their exemption. Never treat a write-up as a license to deduct wages without checking the employee's classification first. A clean disciplinary record, signed and dated, is the single best defense your file can carry into any of these proceedings.