There are licensing issues related to FAT, particularly relating to Microsoft patents that deal with long file names (LFNs).
Licences for long file names (LFNs)
Microsoft announced on 2003-12-03 that it would be offering licenses for use of its FAT specification and "associated intellectual property". The royalty for using LFNs is US $0.25 royalty per unit sold, with a maximum of US $250,000 per license agreement.
Micrium µC/FS is delivered with complete source code for FAT; this includes source code for LFNs. To enable long file names (LFNs), you must set a configuration switch. By setting this switch, you agree to contact Microsoft to obtain a license to use LFNs.