Those who want to give, will ask discreetly. They just thought they'd like to give SGeans the discounted rates, so at their discretion, they may or may not ask. Not ask if they aren't sure, of course, in case visitors turned out to be neither.
Or a place of attraction might have different rates for tour groups and others?
Anyway, not everywhere of course, but it's happened. For me, I won't bother how much each group of people pays, just how much I've to pay, before I decide if I want to enter or not.