The model might differ for copy left licenses, but nothing really prevents it at the end of the day. I think GCC's arcane code base made the situation worse versus business sense. Otherwise Red Hat or MariaDB would be dead.
Does it matter if my competitor can take advantage of my bug fix in a compiler? I don't think copy left licenses are a good fit for every project, but there is certain infrastructure in the world where ideally everyone can benefit.
If you have a competitive advantage why contribute back in any meaningful way? We have seen this type of stuff on Android with Dalvik (where Qualcomm has a special library / modified version of the VM).
That's not to say it destroys freedom, but its not ideal for a certain subset of software. The GPL is also not magical in this regard, since you have projects where things don't get up streamed.