The rules basically say that embassy official should only give out student visas if they are confident that the student meets the terms of the visa, particularly that they have no intent to overstay or permanently immigrate to the U.S. I.e. the default is not granting the visa, rather than granting the visa.
The rules are bizarre: while you're on a non-immigrant visa you have to prove that you have no intent to immigrate or seek immigrant status, yet ultimately many people will seek green cards. Indeed, there is plenty of talk that the U.S. should actively seek to encourage international students to stay permanently, while at the same time the rules say that visas shouldn't be granted to international students who might possibly be considering staying permanently.