Yeah only the first 15 minutes are on their website. No idea why people who own a tv who can watch the whole thing and people who only own a computer only get 15 minutes.
If it's like most other PBS shows, tomorrow it will be available in its entirety. They want you to watch it on your television if possible, then give you the opportunity to see it online after it's aired. It may only be available online for a limited time though, so don't procrastinate too long.