Technically it was because businesses were banned from raising wages during WW2 but benefits were exempt.

War bonds, rationing and other measures long since abandoned were introduced then too. How weird would it be if everyone were still limited to 8oz of sugar per month in 2025 due to the exigencies of WW2? It's no less weird when it comes to healthcare being linked to remuneration. Heck, there are better free market approaches not adopted by the US, since its usually horny for that sort of thing, right up until it's pro-labor I guess.