This was to protest President Clinton's signing of the Communications Decency Act of 1996. The indecency provisions of the act were struck down by the Supreme Court as unconstitutional the following year. Additional legal challenges took ten more years to come to fruition. More detail welcome in the comments on HN.
I wanted to read more about the Communications Decency Act, but Wikipedia is blacked out. It's funny how they scrapped the act, yet choose to repeat history and here we are protesting SOPA and PIPA with another blackout 17 years later.