I never thought about this,

but could it be not the dayhikers that leave WAGs but JMTers northbound who do not want to carry it with them?

This does not seem plausible - for them, the solution is a usual cathole (or under a rock) if needed that one day in the Whitney Zone and keep on going, no bag.

Then there is the difference in clientele, and sheer numbers, so never mind, it is probably the dayhikers.