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For a motorcycle yes, but not for cages...
Your understanding of an alternator is correct for a permanent magnet alternator such as is used on motorsickles--they run all the time and dump excess current back into heat. But we are only talking ~30 amps. The magnet drag is probably 1-2 hp in order to make .5 hp.
In most cars the alternator uses a claw-pole rotor that can be controlled as needed to meet the electrical load demand. The higher your electrical load, then a higher current into the rotor and a stronger magnetic field is created, and more mechanical power is required to turn the pulley. And visa versa for lesser electrical loads--the rotor can be turned off and freewheel.
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