With respect to CEC, there is a difference between a device taking action as a result of aYou also made another generalized statement, "You can't disable CEC via a user interface". Well if you can't disable it you're saying the only way to not have problems is to buy these "adapters" to disable the physical bus. Of course you can disable CEC via a user interface.
CEC request (CEC says power on so device powers on) and a device simply responding/acknowledging the request (CEC says power on so device responds with new power state as a result of the command). Most CEC implementations are all or nothing and both of those scenarios either always happen together, or neither happen.
With respect to the AppleTV, the AppleTV is different. Disabling CEC control in the AppleTV UI disables CEC control but does not disable responses to CEC queries. While this is technically valid per the CEC spec, it confuses a large number of other devices in the chain that expect all-or-nothing when it comes to CEC.