Re the tipping not expected, my guess would be since it is not in all countries or in places where it might not be practical to get gratuity funds in a local currency. And the reviews are what get people to book thru TBL.
I have learned through research in coming across some guides’ non-TBL websites, that the TBL markup is anywhere from 20%-28%, to cover both the TBL percentage they take plus what I’d guess is an imbedded gratuity. It’s why I go lower with any gratuity on those bookings. TBL is great when there is a problem with a guide, so worth the markup in some cases. Other times, if a guide has a lot of stellar TBL ratings, or if it’s a repeat booking on a return visit, I have recently found myself booking direct a couple of times when the guide’s personal site cancellation policy is a good one.