I think it is unethical to hire someone, if you are both part of a special interest network of any kind. If you compare their citation record on (http://inspirehep.net/author/profile/A.Ceresole.1 and http://inspirehep.net/author/profile/A.Strumia.1), there is really little contest who should be hired though (both work in high energy physics, Ceresole more on theory, Strumia on phenomenology). But yes I agree there are many factors that play into the hiring decision we can't know about. In any case it seems most likely that this was his primary motivation.