It's a bit like having a trial of gay conversion therapy and saying 'Due to our selection procedure we may have included patients who did not always expect a conversion therapy approach'. Quite apart from whether such a trial would be ethical, it is clearly wrong for mentally competent people to be recruited into a trial unaware of something so fundamental.
It's therefore amazing to me that these investigators had no problem admitting that not all their participants understood what they were signing up for. But, as others have said, these particular investigators seem to be willing to say out loud what other BPS people tend to only say when they are among friends.
I wonder if legal action on the principle of free prior informed consent might be one way to tackle BPS trials? In a lot of countries there will be legal requirements that people may only be recruited into trials of health treatments with free prior informed consent (unless they are unable to give it, in which case, guardians may be able to give consent for them). I think if investigators had to clearly say what they are really aiming to do (so, not just 'CBT' for example, but 'CBT to change your belief that you have a biomedical problem that is stopping you from living a normal life'), then they would find it much harder to recruit.
I'd really like to know more about this study - what information prospective participants were given, what they were told.