But you knew that.
They're contractually forbidden from "selling their access to it" to arbitrary parties; they can share data only with the consent of their customers, almost all of whom actively want that data shared --- this is a very rare case of a data collection product where that's actually the case.
As such, even if they can contract it such that they are not legally responsible for such use, they are very much knowingly facilitating it. If this was physical goods, rather than data, they would probably been as responsible as their customers.
This is the same situation as a web hosting provider: if it is communicated to them that one of their customers uses their service to host illegal content, then it becomes the web hosting provider's responsibility to remove that content.
Reasonable technical feasibility for the service provider is key here, but it can be argued since the data can apparently be shared in ways that identify OP.
Probably not how the law currently works (don't know, not a lawyer), but I guess it should, as otherwise it allows creating a platform that shares abusively retained data without any reasonable recourse for the subjects of this data to remove the data from the platform.
Californians would have standing under the law but need expensive lawyers to litigate.
AWS has employed expensive lawyers to argue semantics; they host OS VMs and databases. This provides them legal cover for what AWS customers store.
Amazon the retailer stores customer data. A non-customer would have standing under California law to litigate removal of PII should they decide to hire lawyers.
Your reductionism is to law what a Linux beige box on a routable IP, no firewall, hosting a production health database with creds set to admin/pwd1234 is to software engineering.
Coincidentally 1234 happens to be the code to my luggage.
If Flock was just an opaque cloud storage service for law enforcement to back up their mass surveillance to then sure, your argument would have merit; it's not, it's a giant database of photos, locations, times, license plate information, and likely a lot more. They're not selling cloud storage, they're selling (leasing?) surveillance devices and tools.
My experience on HN is that these kinds of discussions almost immediately devolve into debates about what people want the law to be, as opposed to what it actually is.
However, I suspect that is not the case. AWS is agnostic as to the type of data stored on S3, and deletion of PII stored on S3 is the sole responsibility of the AWS customer that chooses to store it.