Just including from the google translated letter reasoning why this was "necessary":
"You were recently included in the study. According to information from some people involved in the study, you have over time […] been active in social media with criticism of a biopsychosocial understanding of ME and of professionals who have such an understanding. You have also been critical of the study in social media. This is of course completely legitimate and it is healthy to have professional discussions.
On the other hand, motivation and a hope that the treatment will work is necessary for a psychoeducational method such as the one used in the study to be effective. Inclusion of people who have no motivation to participate in the study can create a situation that challenges the quality of the PhD work and the PhD candidate's study environment, and can affect the other participants. The consequence of this could be that the study is affected in a negative direction, and as the person responsible for research, you understand that I cannot allow that.
On the basis of possible adverse research-related consequences, I conclude as the research manager that you cannot otherwise participate in the study.''