The NFL is a business, and the network and the league have an image they want, or at least an image they don't want. Not sainthood by any stretch, and a lot can be overlooked or even written off.
But there's a line.
LT has some pretty ugly baggage and even if you believe that he had no idea that the prostitute was underage (I believe him, and there seems to be evidence he asked), he's still a dude that paid to have sex with an underage girl.
Many, many people have that image of LT.