It goes much deeper than just coffee shops and other public wifi. There are people in oppressive countries that have to use VPNs to get around their country-wide bans of certain sites, such as anything that provides access to information. Reddit used to be a sanction for tons of information sharing. But now, with Reddit going public, they have to appeal to their shareholders, who probably have business or other deals in those oppressive countries. So, even if Reddit is simply trying to force users to be trackable, it still behooves the shareholders to make information and knowledge more difficult to access to certain people.

You can still use the site via VPN if you’re logged in. Which is really the entire point. They don’t actually care if you’re using a VPN; It’s just another method to force people to make an account, so the “active accounts” number looks good to shareholders.

They likely didn't. They just don't care.