I guess we will disagree on what is or should be obviously illegal. What your phone does is the functional equivalent of you walking down the street shouting your phone number. It does not strike me as obviously wrong for people to hear it.

I just mean that it's the kind of thing many people who have some concept of how cell phones work might assume is illegal because tracking a bunch of people and storing all that info, or broad, non-tightly-targeted-and-regulated use of things like stingrays, and the various other things service providers and law enforcement do to spy on people really do seem pretty similar to stalking and warrantless search and various other activities that are illegal (so, obviously that would be too, how could it not be, one might reason), and so they might be surprised that a capability they know or suspect exists in the technology is used the way it is and to the extent that it is by both private parties and law enforcement. Their not thinking about their cell phone as a device that spies on them or is otherwise very untrustworthy might not be because they don't know what the tech might do but because they've assumed exercising those capabilities would be illegal.

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.

In a general sense, our problem isn't surveillance but disproportional surveillance. It's generally appropriate for LEO to have access to the same data that all of us do. The problem is when 'privacy' or security laws restrict you and I from data but not the people who regularly exercise their exclusive powers to ruin lives.

the article also goes into the fact that these stingray devices can send text messages from your phone number and listen to your phone calls in real time. I dont think your analogy really captures that, and it's hard to imagine an argument for that being legal (at least without a warrant)