Seinfeld S4/17: The Shoes. Click here to view scene.
For when a man looks down a young woman's blouse it is in the hope of glimpsing breasts; a healthy, heterosexual desire for female flesh. If not quite the done thing within polite society, downblousing is nevertheless regarded as natural and relatively harmless behaviour; something we might even characterize as a bit of good clean fun and place within a comic context.
But when a man looks up a girl's skirt it's in the hope of seeing her underwear; what excites is not the flesh per se and it's for this reason that upskirting is regarded as truly deviant behaviour - fetishistic, unwholesome, indecent, etc.
Thus it is that most viewers laugh when George Costanza stares down the blouse of a fifteen year-old in an episode of Seinfeld and don't see it as an assault by a middle-aged man upon the dignity of a minor, nor a sordid invasion of her privacy.
Would they feel quite as amused, however, had George been caught staring at her crotch rather than her cleavage?