When the child benefit changes were brought in, it was based on one person's income because it was 'too hard' to base it on family income. Yet family income is being used here - have they figured out an easy way to find out what the family income is ? Also, is this a cliff edge - earn one penny over the threshold and you pay the higher rate ?
There are undoubtedly cases where the tenants should pay more - the example of the solicitor above seems, on the face of it, outrageous. But maybe the higher rents these people end up paying will in some small measure make up for the drop in income from the HB caps. Maybe that's why this is being brought in.