It's in the nature of the device you're using: an Apple device. This implies there are non-Apple devices out there who will inevitably fail the check.
It's also in the way Apple allows its use; however that's not as strong. Apple has positioned this as a way to prevent CAPTCHAs from reaching the customer. Apple is interjecting with the provider and saying Hey, trust us. We can prove its a human because they're saying they're on iPhone. They're not positioning this as a way to deny service. Only to speed up access.
