Why are there diodes going to D+ on this alternator/generator
You have a misconception about what the regulator of a car alternator does. It doesn't regulate the voltage of the battery, and so, it doesn't measure the voltage of the battery either.

The regulator is there so the voltage delivered to the battery to charge it is independent from the speed of the drive shaft.

The regulator does this (simply said) by controlling the field winding of the alternator. That's the winding on the rotor, in your schematic that one on the right. This field winding needs some power, and it has to be independent from the battery, so the speed the alternator runs at can be measured.

That's why these three leftmost diodes are there. They provide an additional DC output which is not filtered to "pure DC" by the battery. Instead, it still has an AC part with a frequency dependent on the drive speed.