I find the reasoning consistently maddening. What they should be saying is that there is no federal mandate for private employers to pay workers during vacation/convalescence/maternity leave.
In point of fact, for government workers there is paid annual/maternity/paternity leave which ends up being "use or lose" and in effect becomes a mandatory leave - something universally omitted in these articles.
I think this just comes back to the classic debate about how the American economy is organized. Our constitution and US Code is not organized such that the government can easily dictate laws to private employers, that is by design.