I suppose similar reasoning is how we arrive at our judgements on most questions of legality, personally, when deciding how to behave and what to worry about others doing day-to-day. Like, I definitely can't show you the statute that says driving the wrong way on the highway is illegal, rather than just a very bad idea, and I'm not sure I've even ever been told specifically that that is illegal let alone done the research to makes sure it's illegal—but nonetheless, I'm pretty sure it is. So, I would guess some people surprised that their cell phones spy on them in certain ways are more surprised that they are being used that way and not that they could be used that way.
[EDIT] to stretch the analogy further, if I were surprised to learn that some delivery company had found a way to reduce delivery times by driving the wrong way on the highway, the fact that vehicles are technically capable of driving the wrong way on the highway wouldn't be the part that I found surprising.