Yes, it's part that (someone says "CBR and CBZ are just renamed RAR and ZIP files," and the message gets garbled), but it's also lazy application programming to not, y'know, check to make sure you have the right thing. It's easier on some platforms than others, but software made for distribution shouldn't ever take the user's word for things that the user shouldn't care about.
And I say that as a programmer who's been guilty of getting it wrong.