Your diagram of A/C, Rectified, regulated is a little wring. The A/C and rectified is right. There is no regulator, and models with a regulator have no capacitors to smooth out the DC anyway.
These little bikes rely on the battery to smooth the DC and the alternator has just enough current to keep up. No regulator at all. A regulator can be added. The type of regulator for these does not adjust the output at all, though. The output from the alternator is current, and it has a fixed ratio of current to RPM.
The regulator is a variable shunt to ground, that, when the voltage exceeds 15V, the regulator just adds a load to maintain the 15V. It is merely a limiter, rather than a full fledged regulator. Alternators with variable field coils actually adjust the output of the alternator to match the load. Permanent magnet alternators have a fixed strength field so output regulation can;t happen, only load can be varied to keep the volts down.