I agree.
However, it makes sense to me as it moves the risk of criminal prosecution from the prostitute to the client.
If it was suggested that killing animals was allowed, however eating meat carried a penalty, this would also make sense to me, and I think it would certainly be a measure better suited to end the killing of animals for food than simply putting a fine on killing animals by hitting demand instead of supply. Of course, in the killing animals example, hitting both might be best.