[pavon]

Purely due to industry norms, no laws that I am aware of. For example Tesla makes it much harder for third parties to repair vehicles both by not making parts available and by using DRM/part-pairing/phoning-home to require authorization from them to replace some parts.

[tadfisher]

It's a traditional benefit of decoupling the sales and maintenance of vehicles from the manufacturer. The manufacturer should not care who buys their parts, so it should be purely an additional revenue stream. ASE certification means any shop can carry out warranty repairs and the manufacturer doesn't have to have repair shops and staff in every city/town.

That said, it's not just Tesla that has broken this model; Volkswagen Auto Group in particular are famous for requiring proprietary tools and software packages to diagnose and repair their vehicles, and refusing to sell tools/software to independent shops. I think some manufacturers are just more tightly coupled to dealerships, so there's a profit motive in forcing all repairs to happen through them.

[userbinator]

Also the not-to-be-underestimated culture around car modding and the aftermarket, which has lead to a situation where you can build a stereotypical mid-century vehicle with entirely aftermarket parts, but parts availability falls drastically for anything newer.