The publishing divide

I find it perplexing that the American publishing industry has such a strict divide between YA and adult literature. In a pure capitalist sense this is not productive because by publishing a work in one or the other, you can alienate a group that might buy the book. I'm sure there are plenty of teens browsing the adult fiction in bookstores and some adults browsing the YA but they might sell more books if the divide weren't there to begin with. In a library, there are often titles that make it into multiple sections because they would appeal to multiple audiences. Catalogers make those choices to maximize circulation, so why don't booksellers follow suit? I understand that there are content issues regarding appropriateness, but instead of watering down the YA books that are being put on the shelf, why not make parents responsible as they are in a library setting? In an ideal world all content would be available for all people and it would be parents' jobs to regulate what their children are exposed to rather than the industry doing the parenting. I realize this might be an unrealistic goal, but it has a lot of potential to increase profits.