The idea is that capitalism will lead, inexorably, to fewer big companies and fewer people who control those companies. Those fewer companies and people will have more and more power, the people at the other end of the spectrum (e.g. unskilled workers) will have less and less power, over time.
Wall-Mart has a lot of power; its workers almost none. In fact most of its workers are kept part-time so that they don't have benefits and more than half of the full-time hourly make less than $25,000/year[1]. Remember that almost 60% of Americans don't have $500 in savings. These people are often terrified of losing their job.
The notion of using workers to make deliveries is representative of late stage capitalism in that at first it sounds like Wall-Mart is letting its employees "hustle" for a bit more money, but instead it's really putting itself in the situation where it can leverage its employee's circumstances. Note that, even in this proof-of-concept stage, the risks and insurance costs are already passed to the workers[3].
Please note that I don't think Wall-Mart as a company is particularly evil or would ever try to not pay its workers for this service -- on the contrary, they will certainly, always compensate. My concern is that that compensation will not cover the costs when all variables are factored in and that while it might start as voluntary, if it's indeed profitable, that would start to be pushed toward a work culture pressure of, "Do you want to keep your job? Deliver this at a net loss to you."
[1] https://www.bloomberg.com/news/articles/2013-10-23/more-than...
[2] http://money.cnn.com/2017/01/12/pf/americans-lack-of-savings
[3] https://techcrunch.com/2017/06/01/walmart-begins-testing-usi...