Well, playing a bot (computer) usually asks for a payment, at least for a decent/higher level, which is fair, nothing runs free.
However, you click on an already played game, go tot the "Info" . then click on "computer analysis" then on the bottom click on "Practice vs Computer". Well that gives you highest computer to play (for practice) , and you can also change the computer level if you want, and that is FREE.
So I believe that is a glitch, unless really allowed by the admins to play the bot for free.
The regular engine, which isn't a personality, is open to play by everyone, at all skill levels. It's just the personality ones that are locked to basic members, other than a few.
I want to talk to admin for a possible glitch in the game. Most probably im wrong, but wont hurt if i highlight to admins, where do I write ?
You could post here in the forums and members could let you know if it's a bug or not. Or you can contact support directly.
https://support.chess.com/article/346-contact-us