In wartime, when a government and people are fighting for their survival, many measures are appropriate that are not justified in time of peace. As the US Constitution plainly agrees.
To put it in its bluntest form, which is worse, income tax withholding or invasion and conquest?
The US never actually faced that stark a choice, but then nobody really knew that at the time.
Which is worse: to be an invaded, possibly conquered, people... or to have a government which forces either (A) widespread theft, or (B) every employer to help perpetrate fraud upon its employees?
It's an either-or depending on when the withholding occurs in (a) it occurs when the money is the employee's, and in (b)when the money is still the employer's.
(a) -- Because the money was taken, by threat of force, before it entered his hand.
(b) -- Because the employer is not paying the agreed upon wage.