The idea that the police can simply delete what they don't like or switch of the device when ever they feel like it, is a very false premise in terms of the law. Lost evidence immediately creates doubts about the veracity of statements against those who lost or purposefully failed to gather evidence.
You obviously didn't read the articles. :-)
Simply put, right now today all those bad things are happening. That is what the ACLU was objecting to.
They provided a nice list of actual court cases where officers turned of their cameras on a complaining suspect, and turned them on to a beaten and bloody suspect who managed to injure themselves while resisting arrest.
They provided a nice list of actual court cases where officers submitted evidence of edited films rather than complete unedited films, conveniently omitting the bad things the cop did.
And just look back in the news last month, two cops who had badly beaten a motorist after crashing into him and then working with prosecutors to demand a severe plea deal rather than decades in prison, only to have some cops anonymously post their own dashboard cam showing the police flagrantly breaking the law. The mayor decries that the cops who "leaked" the evidence of corruption and police brutality were "rats", thereby reinforcing the absolute need for the law the ACLU is speaking for.
Specifically, the requirements are the officers MUST record the interactions, the recordings MUST be made available to those involved without editing or deletion, and there MUST be data retention standards to prevent officers from erasing the data too soon ("Police beating yesterday? Sorry, that file was already deleted") or keeping them indefinitely.
Without that kind of law firmly in place, missing video is simply non-evidence; "sorry, my dashboard camera wasn't running" is a valid excuse. Only with that law in place does the missing police video become something judges and juries can rely on.