I get what you're saying and I'm no expert/professional, but i think with anything like this there is an assumption that any modifications or additional parts will be fit for purpose and fitted properly.
I could put 'ebay special' brake and clutch levers on my bike, most likely shorter lever length, made of a different material and possibly a different action. I've asked previously when i had a bike that was massively modified from original (mainly aesthetic) and told anything i didn't mention would be replaced with OEM in the event of a damage claim/theft etc. I had a list of about 35 things
some of which i didn't care for
That said, they don't need an excuse for wiggle room
maybe bike insurers are different, eg you can de-cat and they don't care