Actually - when I was still a Darkside Virgin - I did find evidence (on Metzlers US web site) that whilst legislation does exist in the USA to prohibit the use of tyres labelled M/C on a car rim there was no legislation disallowing the use of a non - M/C labelled tyre on a Motorcycle rim.
There is a reason - NOT ALL M/C tyres are labelled M/C (yet!) - especially older "vintage type" tyres. Most M/C rims have VERY aggressive bead retention lips and M/C specific tyres depend (to some extent) on them being there. This may explain why some guys have had to cut CT's of M/C rims.
In the UK iirc the use of a PV tyre on an M/C is perfectly legal (still?).
Here - The traffic police have never even mentioned it at check stops - and they have mentioned other things - like smoked indicator lenses for example.