They will wear out a little sooner than the flat sheet, because the fitted sheets is the one that gets slept on all the time, but fitted sheets don't wear out at an alarmingly faster rate than flat sheets.
I'm no sheet expert but my experience with fitted sheets, at least the kind with the elastic is that while the sheet doesn't wear any faster than a flat sheet, the elastic gives out long before the cotton fabric wears out.
I also find using a fitted sheet requires each corner of the mattress to be lifted in order for the elastic band to catch and hold. Seems to me a flat sheet could just be pushed under the mattress with little or no lifting.
I would think California legislators would have some more pressing issues to deal with.