The slowdown occurs on systems that can't hold sufficient charge to reliably power the CPU to full anymore. If the battery can't supply the expected voltage, then the system simply shuts off. That is much worse than slowing down. This feature inarguably increased longevity—hardly what I'd expect from a "planned obsolescence" scheme.
They did make a mistake, though: they should have been up-front about it. They should have advertised it rather than hiding it away.
They did make a mistake, though: they should have been up-front about it. They should have advertised it rather than hiding it away.