Please could you provide some clarification as to exactly which other countries allow all the seats to be used on public transport? Surely the UK can't be the only country not to allow this? Your post sounds rather ambiguous without specifying any examples of which countries.
Just checked the websites of RATP (Paris), STIB (Brussels), De Lijn (Flemish region of Belgium) and RET (Rotterdam) and none of them say anything about restricted seating, other than on demand "Belbus" services in Flanders, which have a maximum capacity of 12 for a Mercedes Sprinter minibus, or 4 for a taxi. RET even states "Alle zitplaatsen in bus, tram en metro zijn beschikbaar" (all seats are available).