Ah, you can get round that by opening and saving them in 3D Editor. However, I think it's a lock put on when they've been saved - so if they're not the original cars, the authors probably don't want you mucking around with them
O.K I don't blame the authors for not wanting people mucking around with them, but is it ok to change the files for personal use only, and not releasing them on the internet as my own?