Gotta be pretty low odds that an OnStar module breaks while someone is in a remote location, leaving them stranded - then they make it back, diagnose the issue,

Gotta be pretty low odds that an OnStar module breaks while someone is in a remote location, leaving them stranded - then they make it back, diagnose the issue, take the manufacturer to court, the court finds the manufacturer guilty, and the punishment is substantial enough to outweigh the benefit of requiring the module in the first place. Even if you could hold the manufacturers liable, I'm guessing most of them would still take that risk.