The DVD you are trying to decrypt is probably ARccOS protected. Due to its nature, Fairmount is not able to handle ARccOS protected DVDs. For this type of DVD, you will need to use other decryption tools such as Mac DVDRipper Pro, RipIt, MacTheRipper or 0SEx to decrypt your DVDs.
The reason why DVDRemaster is downloaded when downloading Fairmount is that the latter is distributed with the DVDRemaster package.
Fairmount adds some overhead to the data access. Some applications such as Apple DVD Player expecting low latency access might suffer from hiccups. Since Apple DVD Player can decrypt DVDs, you can simply quit Fairmount and use the DVD normally.
Fairmount does not contain any DVD decryption code. It uses the DVD decryption code distributed with VLC Media Player. Thus, VLC Media Player must be installed for Fairmount to work.
To uninstall Fairmount simply move the application icon to the Trash.