'Why publish rubbish?'. DM: 'DM accepts voluntary contributions from readers, either once-off or monthly recurring basis. Once-off contributions at the discretion of [their?] readers'. 
But it seems to me DM publishes because the writer is ostensibly well-known in his/her professional circle, a professional or budding writer, a 'celebrity' (or what passes for one in SA) or politician and, therefore, in their 'readers' opinion, his/her view MIGHT be interesting or polemical even if it's poorly written. It appears those are the criteria, except on the rare occasion it might be relaxed for Joe or Mary Soap to submit a piece as letter to the editor. I don't know how else one becomes a DM 'opinionista'. Main articles are by staff or contracted writers.
I agree some of what they post is not good. Typically, the drek, ironically, what DM calls 'stupid crap', is by party hacks and mouthpieces.
(I thought they were Jo'burg-based, but they have a Kenilworth, Cape Town postal address.