THE LIBEL ACTION brought by White House aide Sidney Blumenthal and his wife against Internet gossip columnist Matt Drudge has yielded, against the odds, a useful legal byproduct. The usefulness is in a realm far removed from the scandal-whirl inhabited by the lawsuit's major players; it speaks to the providers of online services. Though the Blumenthals had sued both Mr. Drudge and the America Online service that carried him, U.S. District Judge Paul Friedman this week granted AOL's motion for summary judgment to be removed from the case, saying the clear intent of Congress on the subject has been to shield Net providers from any liability as to the content they carry.