When the BBC show clips of what's on later that night, why do they always start with the first programme after Eastenders and never show a clip of Eastenders itself (regardless of what night and time it's on)?
As you've said in your question -- they are advertising what's on "later" that night. Anyway, be thankful. There's enough mindless drivel on the box without adding to it with even a few more seconds of E'enders.
Because they are designed to entice you into watching something you might not see regularly. Those who don't watch Eastenders are hardly going to start now so making little "adverts" for it is a bit like preaching to the converted.