The main reason why Aldi is able to keep its price on milk (and everything else) so low has to do with Aldi's overall shoestring operating costs. Additionally, over 90 percent of the items they offer are store-branded, cutting out the middlemen from their supply chain.
James' Ordinary Guy Reviews conducted an investigation a few years back that showed Aldi milk comes from the same plant that sources milk for Kemps. Essentially, Aldi's milk product is a generic-label version of the exact same product sold at a higher price under a brand-name label.
Another reason why Aldi’s milk is cheap is due to the crisis being faced by the dairy industry. In the U.S., dairy farmers are being paid the lowest rates for their milk in half a century, and over 75 percent of smaller U.S. dairy farms have gone out of business.
It's true that Aldi's cheap milk prices might seem to reflect some questionable ethics, at least with regards to paying dairy farmers. However, with dairy industry profits worldwide seemingly in a state of free-fall, this appears to be a problem too big for protests or boycotts or even switching to a different retailer.