Why would I care if Tim Cook could theoretically look up my resting bpm?

Because a company can tie the timestamp of your BPM reading to your activity on your phone/desktop/what have you and judge your emotional response to whatever you're doing at the time. Psychotic as this may sound, it opens the door for a lot of particularly Orwellian mass surveillance under the guise of targeted ads. They wouldn't be knowing just what you do or buy, they'd know how you feel about it too.