The argument with Stringrays is that even with a warrant to target an individual the police end up sucking up a large amount of random people's location and cell phone data.
Like license plate readers and facial recognition, you're out in the world without the expectation of privacy but I think for most people that feels different when a giant automated system is sucking everything up without recourse.
WHile I don't expect privacy, I want it, and I want other people to really think about it and not want public surveillance as well. We should have some expectation of privacy out in public and not allow a loophole of "oh, but we were looking for someone else but also saw you", it's a huge loophole with essentially no limits. A warrant should cover one "thing" a person, group, etc. Anything else grabbed in the process should not be admissible in court or even be used by the police.
Like license plate readers and facial recognition, you're out in the world without the expectation of privacy but I think for most people that feels different when a giant automated system is sucking everything up without recourse.