And the point being made is that other resources are being optimized for instead - wrinse cleaning uses less effort and thus time. The resource of concern varies by area as well.
Technically a nonpotatable unpurified water tap could be used for the same purpose but in addition to the risk of "oops accidentally drank direct river water with pollution and/or hazardous natrual bacteria" the infrastructure for the fringe use would be less efficient than just purifying more water to be flushed down the drain as a cleaning process.
Plus in say southern California the freshwater purification is not the limit but the input water hence the dirty looks for bottling it there instead of say the Great Lakes area where it is actually abundant. Hong Kong I believe is one of the few places that uses salt water to flush their toliets despite large seaside cities being in no way rare.
It is like mass production technically wasting more materials - at that point it usually doesn't matter compared to the sheer efficiency gains.
Granted non-sustainable uses of source water is something to be accounted for.
