It was entertainment, a big bang for Americans' buck. The fact is, negative campaigning has a rich and tolerated history in American politics. The American people understand campaigning for what it is and pick their candidates more or less on the issues. Whining about negative campaigning is a waste of time. Politicians on the receiving end may even benefit, because most Americans see the attacker as a jerk.
Nobody likes the bad sport, the bully who picks on the other guy.
Psychiatrists who have studied how negative campaigning affects voters have found that the attacker is shooting himself in the foot. Voters see the attacker as hostile, while they view his target as sympathetic. However, despite seeing it as less ethical, a majority find it more informative. Give me negative or give me a sleeping pill. Which is to say, we really don't mind mudslinging all that much.