Doing the dirty work

The BBC's story tonight Lib Dem anger over Brown 'tricks' is a quite outrageous example of journalists doing the dirty work of politicians who don't have the guts to say things on the record. We are told that:

Lib Dem sources said Mr Brown had used "underhand tactics" by going behind the back of leader Sir Menzies Campbell.

Why are the BBC allowing these "sources" a free pop at Brown, behind the cloak of anonymity? In whose interests is it that this accusation, essentially political comment, is anonymised? Was the source Campbell, who repeatedly to criticise Brown in an on the record interview?