Early alternator-equipped Harley-Davidson motorcycles did have this property, as they used a permanent-magnet field and a shunt-type regulator. However, even with these, you could install an aftermarket regulator which did not shunt the excess current, and thus would free up just a bit of power.

But even typical automative generators would control the field current to affect the output voltage, which does cause the input power to be somewhat proportional to the output power (there is always some loss). All automative alternators that I've ever seen do this.