It's exactly what they mean and you're just telling me that you choose to communicate your ignorance by being confidently wrong.

https://equity.osu.edu/education-and-resources/protected-class-definitions

"Prisoners are one of the federally-protected populations of research participants. This status, accompanied by additional regulatory requirements and ethical considerations, is designed to reduce specific risks that may be experienced by this vulnerable population. This includes confidentiality risks as well as vulnerability to undue influence on the decision about whether to participate in the research."

https://www.washington.edu/research/hsd/guidance/protected-vulnerable-populations/prisoners/