[bmh_ca]

The relevant rules for trade wars are "national treatment" and "most favoured nation". You will see those terms quoted extensively in any legal trade dispute, notably before the WTO.

Labour laws per se do not violate mfn or nt, so long as the labour laws apply domestically as well as to imports (since trade wars are essentially about import tariffs).

The primary reason we do not apply labour law to the producers of imports is because labour is poorly organized, under funded, and lacks political will and/or forethought.