I was really surprised when I saw another stock (well, mostly) Android device had this behaviour as the default, since neither my current, nor my previous phone had it. I think my old Samsung S8 did.
While it does prevent a "casual" pickpocket from doing anything meaningful, anyone even slightly more experienced will know to either force power off the phone or shove it in a cheap Faraday bag. If I've understood correctly, the developers consider such a feature "security theater" for the above reasons. Another GrapheneOS-specific argument would be that it needs to be very easy to get the phone into a BFU state. Of course you'll say that this could very well be optional and you'd be right. I doubt it will be implemented unless there is evidence that it provides a measurable security increase.