yeah that's true. a salesperson does get compensated more for making a bigger sale but often it's because bigger sales are more complex and involve more people. My only experience with sales is in software sales and construction equipment sales.
Part of me thinks that the site owner's intent is not to be shady even though it comes off that way but rather they're inexperienced, maybe a younger person, who isn't comfortable or confident with being upfront about getting paid for their work. I see this with greener professionals who don't display their prices whereas experienced professionals will be very upfront and say "this is my price. take it or leave it."