Cafe badging does nothing to address this, as guests don't even have badges to badge with! There's no gate that you need to badge into the cafe to enter, if that's what you're thinking about.

The way to address that is to see who is consistently registering to bring in way more guests to the office than anyone else.

This is generally fine, at least in Google Cambridge.

It's just for tax purposes, as has been made abundantly clear in this thread.

It's generally not fine in other offices where cafes are already crowded just with employees (e.g. NYC). You have to register all guests at the building entrance and if you're registering lots of guests all the time, something might happen. What exactly I'm not sure; I don't know anyone who abuses the cafes to this extent. I think the registration requirements alone substantially curtail potential abuse.