Because prior judicial approval is the key check to preventing government abuse; if police want to shut down a protest a judge potentially telling them they went too far 6 hours later is not going to help.
You're assuming the worst possible outcome (police overreach, court apathy) and using that as a justification to omit any mention of the copious rules limiting official use of this power. I find it rather ironic that you decided to leave your readers poorly informed about this, not least because it would put them at a disadvantage in any argument with a supporter of the bill.
If you're going to shut off several thousand cell phones, you can bloody well get a judge out of bed for that.
For example, the police are only supposed to shoot people when there's imminent risk of death or great bodily harm. Yet there is routine, if not common, overreach there, including one notorious recent incident that people are currently protesting with great vigor.
If you can't reliably enforce the rule that police aren't supposed to kill people except in dire emergencies, how will you do it for cell phone kill switches?
As a result of this, the police will feel justified claiming "imminent risk of death or great bodily harm" any time two or more #insert <target_demographic.h> are together in one place.
Then you just do this often enough that the judges don't have time to review every case.
Don't dismiss as ridiculous what is simply an extrapolation from current practise.