Region encoding is the mechanism that enables motion picture studios to control the worldwide release of their movies. It is required by the DVD Forum (http://www.dvdforum.org/forum.shtml) in all commercial hardware DVD players. Every DVD-Video disc contains one byte of data representing a region code, which limits where the disc can be played.
http://support.apple.com/kb/HT2397
Once you have set the region in DVD Player five times it cannot be changed.
Instead, use VLC to view videos from different regions:
http://www.videolan.org/vlc/download-macosx.html