I would prefer to be told that my battery is weak so I could make a decision on if I want to replace the battery, replace the phone, live with the phone shutting down randomly when battery is low, or continue with a slower phone. That's just me.

So why did they slow down iPhones that weren't shutting down randomly?

To prevent random shutdowns.

Apple absolutely effed up by not communicating the specifics well, but that’s corporate policy. Apple docs have always been targeted at the non-technical user and therefore inadequate for others.