This is not true in the case of buses. They are expected to have priority, with other traffic I agree with you.
The highway code is a bit wordy with it, however (http://www.direct.gov.uk/en/TravelAndTransport/Highwaycode/DG_069858):
Not safe to slam on to give way 3 feet away from it though, is it?
Just because the highway code states buses should have priority when its safe, doesnt mean they should just pull out regardless of whats coming.