When they are part of a group that has a badge option, they can choose to display that with their account or not. The setting is in their "Edit Profile" page.
I believe it was initially written that way (over a decade ago) to be a choice the user needs to make, since not all users want to display an "I donated" or "I'm a subscriber" type of badge on their accounts.
For example, I personally prefer NOT to have an "admin" badge displayed on forum accounts where I have been asked to help test/debug/write/setup features for. I also prefer to not stand out when I contribute donations to a forum on a subscription style basis.
