Apple knew the batteries they had sourced were barely capable of supplying adequate power to the phone under load. As the batteries aged they rapidly got to the point the phones would just shut down unexpectedly even when the battery was showing 50% available charge. This was occurring during a period when the phones were covered by warranty or AppleCare. Rather than replacing the batteries under warranty/contract Apple took the step of throttling performance without giving customers adequate notification. The release notes only had a vague statement to the effect performance management was improved under certain circumstances. Most customers realize batteries are going to lose capacity as they age, but they expect reduced runtime, not random shutdowns. What Apple did was find a way to avoid having to pay to fix the problem caused by their inadequate battery specifications.