It depends how ally you want to look. Me, I wear it on the back of my head with the flap pulled over my eyes like a tweed cap. It looks ace IMHO. I often find bending the cap badge a little so it follows the contours of the material sets it off pretty nicely as well.
I suspect that some wallah thought that the habit of the "Freensh Foraan Ledjionn" of wearing their capbadge over their left (or is it right?) lughole looked really, really ally and started to imitate it. With the width of the Para badge and their wish to look utterly ally in all situations, I suppose it was only natural that they'd head the theme. It's a bit harder to understand why regiments/corps with much slimmer badges decided to follow suit and why nobody pulled them up for it.
Still, as long as the fellasses and fellas are making a marvellous fist of doing what they're trained to do, why not?