That was likely the initial/original reason a few years ago. The lighting control systems in GM vehicles have changed considerably since then. The DRL mode is now just a simple BCM programming change on most GM models now (by the dealer, not by the customer). However, the last time I read the GM policy on making that programming change, the entity requesting the change _must_ be either government or military. Otherwise, no. (That position could have changed)
Over the years GM has been so strongly promoting their versions and statistics touting the benefits of DRLs that they have essentially boxed themselves into a corner on the issue given what has transpired since the introduction. We now have several years of hindsight now...and it ain't exactly what was expected, it seems. Because of that history, they can't _morally_ turn them off, either for ego (save face) reasons or liability reasons or both. GM's competitors outsmarted them big time on this one with statements like "we're waiting on the final government conclusion on them...we don't have a position. But if you, Mr./Ms. Customer want them, we will give _you_ the choice". When one has a product to sell, customer choice is paramount. GM screwed up (in my opinion), they didn't plan for customer choice (and they should have). They also essentially pushed a largely untested DRL experiment on their customers, not only against their will, but without their specific consent in agreement to participate in their DRL experiment!
So, with that, I think the real issue (as of right now) is that there is a significant consumer backlash against DRLs and it is growing over time. Even current non-GM customers that might be "would-be customers" are often found to be "annoyed" by DRLs, especially some of GM's high beam implementations, in particular. Additionally, enough time has passed where real-world long term insurance data is available. From what my insurance company has told me, their "loss data" has not shown any apparent benefit of DRLs in actually reducing "loss" and some of the more recent studies that have been done by truly independent entities (not GM puppet entities) have been largely inconclusive. Other studies (on file with the NHTSA) tie DRLs to safety negative issues. Over time the customer has become more educated over the issue and most simply don't want DRLs any more (if most ever did). Result? GM is loosing sales because of it and they know it. People that don't want DRLs simply can't even consider a GM vehicle at all. Therefore, IF GM can somehow convince the NHTSA to make DRLs mandatory, they're back on even ground because all cars will have them (not just theirs), which then eliminates the competitive disadvantage they currently have, they "save face" too!
Make sense?