Easy! It’s a throwback to the era of sidecars on motorbikes. These are mounted on the right hand side, so they make it easy to remove the right footpeg to attach the sidecar when you want to clip it on and easy to replace when you remove the sidecar.
The left peg stays put, so is riveted.
For example: https://www.cyclesidecar.com/attach-to-enfield/
Ahem: this may be absolute BS, but is good a theory as any
Accurate or not it sounds as plausible as anything... I salute you and from this day forward i will go with your explanation when the subject comes up.