Do you wonder why the file are read only? That's because they are NOT TO BE EDITTED! The CORRECT way to add boot options is to run:
gksudo gedit /etc/default/grub
edit:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
to:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset"
Save the new file, close gedit and run:
sudo update-grub
The files are GENERATED using the config files, so by using your method, as soon as a new kernel is installed it will UNDO THE CHAGES to the file. Please advise users correctly, or don't bother. The community has enough to do and doesn't need to help dig them out of issues caused by misinformed users like yourself.
I suggest you change this so that it is CORRECT.