Probably for the same reasons dogs chase their tail and Republicans call for tax cuts. It's what they do, so it's got to be right. Here are some taxes, let's cut them! You're in a commercial, put on a bikini! Overtly sexual advertising is everywhere, but it's the Laffer curve of the marketing industry.
It's interesting to me that the more successful companies, including food, rarely go the route of "let's seduce/confuse our consumers with sex." McDonald's commercials are basically a United Colors of Benetton ad, with french fries. Coke Zero commercials feature middle-management types carrying on about the legal liabilities of their product. Subway ads are a jingle turned into the song that never ends. ("Five..")
But not Hardee's! Along with their sibling chain Carl's Jr, Hardee's is a decently profitable company (especially in the recession) that still has to shout to be heard over its more beefy brothers McDonald's, BK and Wendy's. Except instead of yelling, they're making balls jokes and asking reality stars to roll around in the sand with their burgers. Strategic marketing! Let's watch and cry.
(Editor's Note: sort of NSFW, but they'll be on TV anyway, so the S stands for suitable)