I still don’t get this. As far as I can find, Steam doesn’t allow steam keys to be sold cheaper elsewhere, but they don’t bother with prices of games in other stores.

This is tricky. Officially Valve doesn’t have any rules about game prices on other stores. Unofficially evidence has been put forward by way of emails between developers and Valve that seem to show that Valve unofficially requires price parity with other stores and will punish games that offer lower prices elsewhere.

The charitable interpretation is that their policies are worded confusingly and some of their agents are misinterpreting the rule requiring Steam key prices to be uniform as applying to non-Steam keys. The uncharitable interpretation is that Valve knows such a policy would get them in hot water with anti-monopoly laws and so they’re careful to make sure it stays an unofficial policy.