I simply think that a an explicit backdoor in Intel ME would be quite a risky move, but I do not have access to non-public information, so who knows?
Also, the ME plays a fundamental part in the initialization of the Intel silicon (especially from Skylake/ME11 it appears), so it makes sense that it can not be disabled freely. It is close source because it is the way it works in the silicon world, as having an open firmware exposes information which may be bring advantage to the competition.