Because I think they're being terribly clever about this whole DEC row. I'm in two minds about their decision that it would undermine their impartiality, but I don't think you can say that they institutionally don't want to help, because, funnily enough, the controversy means that they can happily
report on themselves (and provide a link to the DEC homepage as they do so).
I mean, I was just on the BBC's homepage, and in the little window-y bit (where they have interesting bits of news and programmes) the "BBC Charity Appeal Row" is one of the things you can click on, which puts a link to the DEC right in front of you, which in turn immediately starts telling you to to "donate now". All right, so maybe it's not as much as a prime-time telly advert, but it's still pretty damn good publicity.
And not at all undermining the BBC's impartiality. :P
Quinara.