I don't think it's prostitution at all. Surrogate therapy isn't focused on sex, though sexual activity may be a part of it for some clients. It's about working with a person to become comfortable with her or his body, with physical intimacy (not necessarily sex) and sensation, with emotional intimacy, and, possibly, with sexual experiences. People who seek the services of a surrogate include people with physical disabilities who may be unfamiliar and/or uncomfortable with being touched by people who aren't medical professionals or caregivers. Also, people with a trauma history can benefit from the services of a competent, trained surrogate to address the anxiety they might experience during intimate encounters.
What surrogates do and what prostitutes do are two very different things, though they both may have sex with clients for money.