I'm against exploitation of workers, too, but at the end of the day there is only so much you can do with jobs that require relatively unskilled labor. If a 15 year old can do your job as well as you can after a few months of training, it's not a safe job that you can expect anything out of. It's gonna get automated away anyway.
But here Walmart (whatever their other faults), offers a way for workers to make money doing something they already have to (thus it has more utility than their normal role), and they're evil?
If people's regular jobs paid a "living wage", people wouldn't have to Uber, but it's how the world works.
Kind of damned if you do damned if you don't isn't it?