Phrase: Judge a Book by It's cover

For years... the phrase:
"Don't judge a book by its cover...
Because you could miss out on an interesting read..."
Has been used as a way to tell us all, not to negatively judge someone by first impressions..

But why don't we use it the opposite way too???

When I say:
"don't judge a book by its cover"
I'd mean:
"Just because theres a women in a bikini on the front page doesn't mean 98% of the pages wont be advertisments for men boxers!!!
Which basically means, youve not.purchased a mag full if Tits, but a mag full of men's junk!!"
Oh the irony...