See DMCA and licensing agreements. Modifying your stuff might be a felony someday. Best to be sure the right to modify or repair is right there in the license. Irrevocably and transfering to next buyer automatically. Not happening right now.
I seem to recall that a similar argument was used against Jon Johansen at some point, but I don't feel like trying to dig up early-2000s history on the web today (IME going back to ca. 2003 is okay, but before that a lot of things have fallen down the memory hole).
[1] http://volokh.com/2011/01/13/todays-award-for-the-lawyer-who...
https://www.eff.org/wp/unintended-consequences-seven-years-u...
In a similar vein, if I were to physically modify the engine in such a way as to make it faster but fail environmental or safety regulations, that could also have financial or legal repercussions for me (or Volkswagen, to cite a recent example :). The same argument could be made against modifying automotive software- vehicles are certified by government agencies around the world based on the idea that they will perform a certain way WRT safety and emissions, therefore software behavior has to be just as unchanging and predictable as hardware behavior. The fact that people can and do hack their cars all the time doesn't change the regulatory environment for car companies.
I do believe that embedded software should be as open, modifiable, and testable as possible, but I don't think people claiming the right to tinker as a basic human right are going to change anyone's minds about this. The biggest impediments to (for example) car companies making their ECUs open source and hackable have nothing to do with a dogmatic attachment to proprietary software and security-through-obscurity (although those concepts have their adherents, misguided as they may be), and much more to do with compliance with government regulations and minimizing their potential liability.
There is at least a perception that having open source, hackable ECU software would be all downside with no upside to the companies selling those vehicles. The way to change this perception is to show it as being demonstrably wrong, rather than to simply claim a right to tinker. The best way to get open source ECUs is to either make the consumer market care about this (not likely) or to update the regulatory environment in such a way that car companies have a non-abstract motivation to go open source.
This is not unlike the dreaded binary blobs required to use certain 802.11 chips on open source operating systems- they exist because of FCC requirements, not because the vendors love binary blobs. They are allowed to ship software-defined radios, but the only way they can guarantee that their SDRs don't broadcast on illegal frequencies is to lock down the software that controls the hardware. It's not an ideal solution, but its the only solution they've got aside from spending even more money to ensure that safety at the hardware level, which wouldn't do anyone any good.
It's a sensible counterpoint. The modification would be, if it's safety-critical, that you have a right to get modifications by professionals who aren't the original seller. Or perform them yourself if you are such a professional. This covers the biggest use-cases of repairs and enhancements respectively.
EDIT to add:
" is to lock down the software that controls the hardware. It's not an ideal solution, but its the only solution they've got aside from spending even more money to ensure that safety at the hardware level"
Not true. They could do it right at hardware level with simple bounds-checks burned into logic. The checks' setting set at manufacture or OEM configuration using something like anti-fuses. The kind of tech that's in 8-bit and 16-bit MCU's that retail for a few bucks. Cost almost nothing. Also more secure given simplicity. They aren't doing it because they don't care given demand side of this issue. Plus, lock-in among oligopolies brings them long-term financial benefits. ;)