An alternator is regulated, but how it’s regulated is what makes the difference. The alternator’s comprised of basically four components, the armature, the field winding, the diode bridge, and the regulator. The armature generates an AC voltage that’s dependant on rpm. The current, however, is dependent on the magnetic field that it sits in. Since the final output voltage is a product of output current and load, it’s then dependent on that magnetic field. The regulator then adjusts the final output voltage by adjusting the current through the field windings, which determines the magnetic field that the armature sits in, which determines the output current and, in the end, the output voltage. The problem with disconnecting the battery lies in the fact that it takes 2-3 thousandths of a second for the magnetic field generated by the field windings to collapse, so even when the regulator shuts off the current, the armature output current remains what it was for a short time. The battery is a very low resistance source, so as the load changes, the battery keeps the voltage stable.
Keep in mind that although 2-3 thousandths of a second seems to be a short period of time, it’s excruciatingly slow when compared to most electronic circuits, and although it may not damage the EEC immediately, the alternator can easily spike to 120V and exceeding the breakdown voltage can very well lead to a failure at a later time du to damage to one or more of the P/N junctions of the components.