Diodes have other ratings apart from current. The important one here is PIV raing or peak inverse voltage rating,....which is measure of how many volts from an exterior source connected in a reverse bias way the diode junction can handle before it punchs through,..which is how the power diodes in an alternator would see a battery charger ie inverse or reverse volts.
Now about here you may be thinking "but the battery charger is not higher than the alternator in voltage" true, but, there are things called 'voltage transients' which are fast spikes of voltage which can easily exceed the PIV of a diode or any solid state device. Transients can be generated by inductive appliances (vacuum cleaners, or anything with a transformer or motor *when its being switched on or off* on the same house-mains circuit. These transients then flow thrui the primary and sec windings of the charger, past the bridge, then into the cars electrical system.
When manufacturers build things like alternators,..they will use the power diodes which are the cheapest for the job at hand. I-max rating is considered, but PIV rating is a throw-up.
Jason