No, the employer isn't paying the hourly employee for their time per se, but rather, for the value that they think the employee can provide in an hour. So if the employee can still provide the same value, even while multitasking, then the employer still gets the value that they paid for.

The bottom line is always about value - and time is only ever just a somewhat useful, but also imperfect, way of estimating value. But time is never the value itself.