Your absence of concern is, as I said, dismaying. If there is a flagging system and yet these adverts are still proliferating, then the list is not being regulated enough. The Tribune story indicates that this is far from a couple of isolated instances.
Perhaps you would like to read a report from the Joseph Rowntree Foundation expressing concern that even the best-intentioned online lettings sites might drive segregation. It was talking in terms of wealth, but it's clear the principle extends to race as well. Or at least that things are heading that way.
http://www.jrf.org.uk/pressroom/releases/170805.asp