Why would you want to put the onus of proving something illegal was done on the victims?

Because someone is innocent until proven guilty maybe ?

Banning pervert glasses solves the problem at the root. Plus it’s orders of magnitude more easy to prove someone wore them than proving something illegal was done with them.

Maybe, but this way you ban them also in situation where they could be usefull.
In a way you are punishing a technology instead of the people making bad use of it.