How working unpaid hours became part of the job

A fair comparison of work in office vs. WFH/remote would be to count the daily commute as 'unpaid hours'. After all, this is time spent for the purpose of the job and nothing else, and it's not paid (and actually costs you financially). If you WFH this disappears so you could potentially both work more hours and still have more time to yourself.

Regarding working time itself, for many highly skilled jobs (article takes an example of a law firm) everyone knows that you are hired to put unlimited hours as may be necessary and expected, so the whole argument about unpaid hours becomes pretty much moot.