Difference between amplitude modulation and intensity modulation

The usage difference reflects the essentially noise-like nature of laser light relative to the modulation rate. The word intensity means average power, that is the variance of instantaneous amplitude, and is preferred when the underlying modulation is very slow relative to the natural fluctuation rate. The average instantaneous light amplitude is zero and in that sense it is just like RF thermal noise and it stays that way even when the mean square amplitude, ie., the intensity is modulated.

In amplitude modulation of a sinusoidal carrier there is a non-zero "amplitude" that is being changed at some rate much smaller than the carrier frequency. This also modulates the intensity, ie., the average power of the carrier but from an engineer's point of view it is usually done by a multiplicative process (heterodyning) instead of source bias modulation or by a variable attenuator as is more applicable for a laser source. Both bias modulation and variable attenuator can be used in RF but they are noisy and rather lossy when compared to mixing/heterodyning. Making a laser source somewhat noisier by bias modulation makes no difference in practice, not so for your AM radio...