You have no idea about the US visa system. A few years back, my B1 visa was put on hold for 6 months, just because of some administrative formalities. And many of my friends have faced difficulties with different types of visa. If you are a citizen of some of the developed nations, then it is relatively easy for you to get your hands on a US visa. But if you are from countries such as India and Pakistan, then it takes a lot of background checks and additional verifications to get a visa approved. In short, having a valid ticket doesn't guarantee you a US visa.