the source of the story was NPR-- hardly the Fox News Network. The story mentioned that it wasn't just Hillary that was served but a bus load of campaign workers at the same time. The owners of the store didn't want payment for the meal. Normally when a large group is served a tip is added automatically, but because there was no bill maybe this is why the tip was overlooked. In any case, the detail of leaving a tip is undoubtedly the responsibility of a staffer and so if you want to ding Hillary for this its for having a staff that doesn't pay attention for detail. I will bet that the waitress receives a tip in short order after the story.
UPDATE: (From NPR)
Editor's Note: Since this story aired, Hillary Clinton's campaign contacted NPR to say that the campaign paid Maid-Rite a bill for $157 the day of Clinton's visit and left $100 in tip money. NPR contacted Maid-Rite manager Brad Crawford, who confirmed that a bill was paid and tip money was left. Crawford, who was not in the restaurant at the time, said that he believes a campaign staffer left the money with one of his employees, but "where Hillary was sitting, there was no tip left." Neither Anita Esterday nor the manager on duty that day were available for comment as of noon Thursday.