Nikon being a corporation and it is a for profit organization, I'd imagine the only way to make them change their mind is if anything we do actually affects their earnings or else being forced to change by law. We can sign all the petitions we want but until we start to speak by spending, such as not buying their cameras and lenses, there is no intensive for Nikon to change its mind. I doubt sufficient number of people will stop buying Nikon gears to make sufficient difference.
I don't know for sure, but doesn't automotive industry have a federal law requiring manufacturers to supply parts to indy repair shops? Perhaps that's a better way to approach this. Petition for legislative change.
This kind of practice isn't limited to Nikon. Some of the higher end companies are much worse. For example, Rolex won't sell anyone parts. Not only that, if they find the watches they receive for repair has been altered by a third party - even if it contained genuine parts like dials, they'll change it back to how the piece left the factory before they'd start the repair. If it contained any non-genuine parts, it will redo all the repairs before they'd do theirs. Yet, they do very well selling their pieces. Third party repair shops are plentiful too - despite these restrictions.