It's allowed in some German cities. With a lot of delivery people being Eastern European, is it perhaps the norm for it to be allowed there too? You just put it in the multipurpose space and hold onto it, though if a wheelchair user wants the space they have priority.
It's not allowed anywhere in the UK I know of (a few bus services do take bikes but have dedicated space for them) but I can see how someone not from the UK might think it was because it was in their home country.
On the other hand I'm amazed they got past the driver with it on a Bee bus - in London it'd be easier as they could board at the middle and the driver might not notice if they did and it was busy.