There are considerable uncertainties in the legal situation with respect to the position of the US movie studios. Certainly illegal copying of DVDs is to be avoided, but unfortunately the CSS copy protection scheme (used on DVDs) has become a view restriction scheme, thereby, according to many, restricting freedom of speech. Thus the required library to read many commercial DVDs needs to be obtained separately.
# sh /usr/share/doc/libdvdread2/examples/install-css.sh
Also, to allow users access be sure they are in group cdrom.