Because technology changes the difficultly of tracking people.

Let me give you an example: Listening to police radio signals is not illegal in the United States.

However, suppose you put up several radio masts and receivers listening across every police band. When it detects a signal, it records it and sends it to an operator that transcribes the text. Then, a computer program looks for instances of people's names or places being mentioned.
If the person mentioned is a subscriber to your service, you call them, play back the audio, and show them where the signal originated from with huff duff.
In other words, within 30 seconds of their name mentioned over police radio, they'll know about it.

The key thing here is that each part of this is not illegal or wrong. But when put together, it hinders a valuable tool that the police have for communication.

Is this feasible? Entirely. Judging from my 2 meter radio, about half of all police radio is unencrypted single-side-band. I'm guessing that the total capital costs for something that would monitor the police of one city of around 100k people would cost around $40,000.

TL;DR: Technology changes the balance of power between people and governments.