Well spotted. I wondered how long it would take for someone to notice.
Lots of reasons why someone would choose the anonymous option.
It's a feature available to PIE people (set it in your profile settings). Anyone can buy PIE subscription (one slice lasts one month) and the money goes towards supporting permies. One can also earn PIE by making exceptional posts and people give you PIE slices. So yep, it's available to everyone.
We really need to update the PIE feature list. There's a lot of new features available for PIE people.