Apple throttled devices that had a weak battery, because the alternative is the CPU trying to draw more power than the battery can deliver, the voltage sagging, and the phone rebooting.

By itself, this throttling is a good thing and keeps phones usable for longer, because a phone that is slow is better than a phone that randomly reboots.

The problematic part was that they a) didn't disclose it, and b) did this for phones within the warranty period, so instead of the phone visibly crashing and you returning the obviously broken phone, it just lost performance which you might not have noticed in time to get a free replacement.