Ask HN: Why do companies put captchas on checkout forms?

During business trips I've noticed companies such as United put a captcha on the form where you enter your credit card number to purchase wifi... Also, there are a few other places where I've seen captchas on the credit card checkout form. Why do companies do that? Is there a good reason for it? Or, do they just add it because they've seen it on other forms and "forms are supposed to have captchas, right?"? To me it seems like a great way to reduce conversion rates for zero benefit.