There seems to be a common belief expressed repeatedly in the comments here that if teens are informed about birth control they will use it. Anyone who has ever been a teen, known a teen, or had a teen-aged child knows that information and application are not necessarily linked. In fact, the number of adults who are informed but lack follow through is enormous. So we cannot know for sure that Bristol (where did they get these names, anyway?) and Levi did not know about contraception or where and how to obtain it.
And even when used properly, all forms of contraception have some percentage rate of failure. This is why abortion is legal, because even the best contraception sometimes fails.
Given the wide publicity sex receives in the media, I would find it extremely unlikely that these two kids did not know how babies are conceived, or how to avoid it. Anyone who believes in evolution knows that we are very strongly compelled towards reproduction. So why is everyone surprised?
As the Palins said, "Life happens."
