The intent of the law, which dates to the 1960s, is to protect the NFL's television ratings by preventing large crowds from gathering to watch games in public places — where their viewing habits aren't measured by the Nielsen ratings. (The ratings only measure viewership at home.) Sports bars and other businesses that rely on televised sports to draw patrons are exempt. |LA Times via Drum|
So the reason that it's illegal to have a superbowl party is that such parties make it more difficult for television executives to track viewer behavior. One can only hope that improvements in surveillance technology will someday make it possible to relax these lamentable restrictions on the scope of our liberties.