Perhaps the FCC used the following reason for setting the Part 15 AM power cap at 100 mW to the final stage.
This amount of power with a 10-foot length of wire antenna provides a useful signal to an AM radio located in close proximity to the transmitter. Any more power causes some nearby radios to become overloaded and thus distorted.
The "close proximity" purpose of Part 15 AM has origins in the phonograph oscillator, the original form of Part 15 transmitter, intended to inexpensively input an audio signal to the superior amplifier, loudspeaker and wood cabinet of an upright AM console radio.
I have proven the liklihood of this theory by use of an AMT5000 from SStran.com, which has the convenience of a fine-tuning adjustment for the power output.
When set to the lowest power 0f 36 mW to the RF input, the AMT5000 rides the background noise on the AM dial at night, resulting in other stations creeping in and mixing with the audio.
When set high, 275 mW to the final RF stage, a radio 10-feet away distorts due to overload.
But the mid-setting, 100 mW, presents a solid undistorted signal to radios near and far within the same building.
A further use of Part 15 AM transmitters is a matter of clever invention by users, who have discovered that by placing the transmitter/antenna outdoors with excellent ground radials, a 100 mW signal can cover a distance exceeding a mile, according to many reports.
My Case for 100 mW has not been refuted, proving its truth.