bkuhn one issue is that the GPL is 3 or so licenses that are similar at a distance (2 of which are widely used)  while as the FSF itself has noted, 'proprietary' covers countless licenses (all the more so if 'proprietary' is used to mean 'nonconformant to the Free Software Definition'). Easy to see how a particular user might prefer a particular proprietary license to the GPL even assuming absence of superstition, irrationality, etc.

On the issue of cause of action for upstream inclusion of GPL-incompatible code, perhaps he is thinking of implied warranty of noninfringement (see e.g. UCC 2-312; cf. copyleft-next 0.3.0 sec. 12).  Breach of warranty claim not inconceivable; seems to be commonly assumed that the warranty disclaimers found in most free software licenses (which typically don't explicitly exclude non-infringement) are sufficient.  Under US law though we're already assuming a lot, including that the company was sued for copyright infringement by the other copyright holders, i.e. the copyright holders of the GPL-incompatible code. Kind of far-fetched, to say the least, but I wouldn't say illogical or uninformed necessarily. Slightly more conceivable if the non-GPL code is proprietary (usually when we talk about GPL compatibility we are assuming a free software universe).