Socialists like to imagine that companies are shortsighted and only wanting to make a buck in the next 10 days. Aside from the fact that it would probably take years to pay off any capex, there are many sources of regulation in the economy.
Intermediaries serve as one source of regulation. These are the businesses who act as middle men between producers and consumers. Even if there were a business who wanted to mislabel cans, Wal-mart would never sell consumers a mislabeled product. They’re in it for the long term.
Also if you had a contract with wal-mart to not sell you mislabeled or poisoned food, it would be enforceable on a free market. To be more competitive an attract more consumers, walmart execs could easily draw up a general form, sign on the dotted line, and everyone’s covered.