That's on the BBS itself. E.g. a regular account might get an hour per day whereas an 'elite' might get 4 hours per day. BBS' were of course run through phone lines and while many operators set up multiple concurrent lines, access was still relatively limited so time limits on the site were necessary. Donors, by contrast, enabled operators to expand their service and buy even more lines and so on.

The games themselves are a different story. It's of course impossible to prove a negative, but I certainly never experienced any paid cheating or favoritism in Trade Wars - which I played on dozens of different sites and later on on the large telnet sites once the internet became a thing.