'Bog Standard'

This expression may derive from commerce as a very recent corruption adopted by journalists of 'bulk standard', rather than being a corruption of 'box standard' (meaning the standard of an item newly purchased and taken out of its box).

The commercial meaning was that goods purchased in bulk should by law correspond with the standard specified in the contract or with the agreed standard in a previously agreed sample or description ie. be of 'bulk standard' namely that the bulk would correspond with the agreed contract description or sample.

You asked: And why do journalists use such appallingly poor English anyway?

Language skills are on the decline in general. A second factor is that journalism has gone to hell in a handbasket. Pretty faces and "happy talk" have taken the place of good journalism regarding television news in the U.S. Print journalism isn't much better.