It is talked about that SPOs should have the ability to decide with whom to do "business". The problem is, that a delegator is NOT a customer of the SPO. The SPO is offering a service to secure the protocol. SPOs are not in contact with delegators and that's for a good reason. It makes things at the base layer easy. Not having the choice is much less of a burden than having it. CS makes SPOs vulnerable because they had the choice to refuse and may be accused they did not do their due diligence.