Not just copyrighted. Microsoft claims a whole bunch of patents related to exFAT. This means you can't legally implement exFAT without a license to those patents, even if you've never even seen the Samsung code. This is the reason why exfat-fuse is not part of any large Linux distributions.
It's also most likely one of the reasons exfat even exists: so Microsoft can exercise some control and have licensing income from a filesystem format being in common use, which they don't have/lost with FAT.
