That is, in fact, a business call. Having said that, the best RPO agencies nowadays always try to always work directly with hiring managers. In fact, the reason why they pick hiring managers is that they’re capable enough to maintain a healthy and a working relationship between the candidates and the hiring manager.
But again, who works with an RPO partner is a business decision. RPOs typically work with in-house recruiters when the hiring manager isn’t available or in situations where mass hiring needs to be done (which involves a lot of candidate processing) in the shortest time possible. Even if the in-house recruiter is the point of contact, the hiring manager must always be involved from the get-go as it can free the in-house recruiters, who can divert their attention to more critical tasks and focus on understanding how they can leverage their RPO partner’s services for their benefit.