When current is running through the alternator, it acts like a motor, generating torque in the opposite direction to its driven speed, which has to be overcome by the driving engine and in turn loads it. If it didn't, you'd be getting power for nothing.
I understand what you're getting at but I really wonder/doubt if it's really that significant. If you were to take that alternator and spin it with your hand, you would generate some electricity. Put it under a load, you're saying it would resist your ability to spin it. I don't think it will, but it would be a good experiment.
Either way you're not getting power for nothing. You're expending gas to recreate that power.