You don’t see the harm in requiring telcos - famous for handing over data without warrants or court orders - being forced to have identifying data for every subscriber?

I can think of a half dozen ways that can get abused. Remember that in the states policing is decentralized. There is always some department somewhere willing to abuse their power. Look at how flock has been used to stalk partners, or how geofencing was used to sweep up everyone in the area of a protest, or how stingray is used to listen to all calls in an area. This is opening up avenues of abuse for almost no benefit.

> famous for handing over data without warrants or court orders

More concretely, famous for supplying bulk data to the surveillance industry for a nominal fee. That is ostensibly the goals behind this development - all of these companies demanding phone numbers for "verification" and snake oil "2FA" want to reliably dox 100% of their users rather than just 80%.