I can think of plenty of reasons, e.g. NSA, or any US institution interested in intelligence gathering, paying for it.

But why would the do it in hardware when they can do it in software? Again, the number of people running Asahi Linux is minuscule.

I dunno about such an exposure tanking their stock. Sure, some privacy advocates would be disappointed, but most Apple users would shrug it off. I doubt a significant percent of people using Apple products are doing it because of privacy concerns. And what are those people going to do? Switch to Microsoft? If they were ever going to switch for privacy reasons, or had any willingness to switch to Linux, they probably already would have. But it's all speculation, because getting statistics on this would be almost impossible. My opinion is, it might make a temporary blip in their stock price, but there'd be no enduring impact on their bottom line. Adding hidden telemetry to their chips, however, would add significant cost to every chip.