I've been the passenger at some arbitrary stops. "Road tax" stops happen to farangs in Thailand outside the cities all the time. You're supposed to play dumb on language but you can try to bargain the fine down (B100 or so). And you can't get a receipt or the price goes up, and you definitely don't want to go higher in the chain or the bribes would get bigger.

Also was in a bus in Vietnam that got stopped for a "fine" because they saw foreigners in the bus. Apparently we weren't supposed to ride the ordinary buses (the ones with livestock, etc.), but we were on it after the bus we were on broke down between Hoi An and Hanoi.