30 years ago, I managed a Burger King, and even then, they had us send people home if it got really slow, or.. call up people and cancel their shift an hour before, because of bad weather, etc. Someone's already arranged childcare or transportation for their shift, then cancelling it same day is just not a way to work.
I would imagine that a lot of places are still like that today.
$15/hr sounds great, but if it's 20 hrs one week, then 7 the next, then 24, then 4... that's still a bit unstable for a lot of folks. For someone who has no dependents, or has a stable partner... it's probably doable.