Meh.
I'm against forcing the auto makers to do much of anything. If this becomes a serious problem, the market will fix it - proprietary software makes the cost of ownership go up, fewer people buy that brand, brand goes under. Isn't that how the free market is suppose to work?
Further, I don't understand how proprietary software could be required to make a diagnostic/repair. Every interface between mechanical car and computer is either a sensor or an actuator. And the test procedures for these sensors and actuator are already listed in the repair manuals, which I'll note are generally aftermarket products as well. [Should manufactures be required to publish their factory manuals? Why? And why not before now?]
To me, this looks like an attitude I generally see in older people. "Wait, I need an o-sill-er-scope to test this part? I don't know how to do that!" Yea, the tech is changing. And that means the technicians will need to get new training so they can understand electronic systems. It's the future! Deal with it. No, you can't fix a car with a socket set and a timing light anymore - you *need* electronic test equipment; but it's already commercially available – oscilloscopes, multimeters, probably eventually logic analysers. Hell, we already have a standarized diagnostic interface that is mandated by law - OBDII; that dumps out of what the engine computer thinks is wrong.
In my mind, this is about technicians being butt-hurt that they have to learn new skill sets and acquire new tooling, and would much prefer if the government forced manufactures to provide a no-sharp-corners way in. I mean really – do you expect to find a schematic when you take the cover off your LCD TV? Should Apple have to provide a way for non-factory techs to get to the diagnostics console of an iPhone?
Sorry, forcing companies to do things at gunpoint doesn't make the world a freer place. I don't support it.