Why does Unix set the executable flag for FAT file systems?

I've noticed that when I mount a FAT filesystem on Linux, all of the files have their executable permissions set. Why is this? There's almost no chance that you can or want to directly execute any program found on a FAT file system, and having the executable bit implicitly set for all files seems annoying to me. I understand that FAT (and other filesystems as well) have no mode bits, and so the 777 mode I'm seeing on files is just simulated by the filesystem driver under Unix. My question is why 777 instead of 666?