I understand they have certain criteria for who they take on board.
Probably they have a checklist of items regarding the website presented and if they are all met the aff is in.
Like any such thing, it’s going to not be perfect in seperating the grain from the chaff and people will undeservedly fall through the cracks.
That’s likely why they avoid certification, probably they would end up having to justify decisions and get into lengthy arguments.
It’s really their business how they pick affiliates for their program, and if they make some bad decisions on that level it’s them who loses out.