I think we're talking about different things. If a manufacturer wants to eavesdrop on my device then they need to push a patch to my device. They can only get information about what happens after they push this patch. They can do it at the start of the life of the device, but they'd have to do it then.

However, if important data is retained in logs, then the manufacturer could grab the data from the logs. They can get information from a time before they decided to look into you.

It's like a wiretap vs access to a diary. A wiretap only gives you information after the tap has been installed, whereas getting your hands on someone's diary would give you access to previous information too.