They leave no doubt to your commitment when you are signing.
However, the academies realize that a lot of kids don't fully comprehend nor the fortitude required to be an officer when first entering.
This is why the academies have no long term requirement until you begin you junior year; then, at that time, each have their own special commitment signing ceremony.
My son is at the USMA. He actively sought his appointment from our Congressman and both our Senators (the Congressman came through). The Army was very very clear that a five-year active duty commitment is part of the deal (in fact one army football player, Class of 2008, was drafted by the Lions but had to skip that opportunity). And yes, cadets can leave with pretty much no strings attached all the way up to the first semester of junior (cow) year.
I have always wondered though, about such grads as David Robinson and Napoleon McCallum - how did they work around their commitments to play pro ball? The Patriots have Kyle whats-his-name (fullback and special teams) on their roster, and he graduated the Naval Academy in 2006 I believe. How did he work it out?