Regardless of the rules, from personal experience I can say that in practice one individual officer can indeed arbitrarily block your entry when they are having a bad day.
A german friend of mine was accepted at a US university, and went to the Embassy in The Hague (Netherlands).
She had prepared all documents to apply for the visa and after having them reviewed, the officer (a woman who seemed to dominate the department) simply said No.
Upon asking what the problem was, perhaps whether any documents were missing she simply answered:
I Just don't believe that you are going there to study
After this arguments did not help and my friend was left speechless.
She contacted the university, got a letter of recommendation from them and they helped her get a new appointment.
This time procedures made sure that her case was not handled by the same person. The officer helping her was less intimidating.
Whilst reviewing the documentation all seemed well, until the dominant woman appeared again, gave her a quick look and simply said No.
After this the guy handling the case looked very embarrassed, stopped examining the documents and rejected my friend. After the dominating woman left he whispered:
I'm sorry but you shouldn't apply here again. Perhaps your school can make the visa for you
So, to conclude:
Even if you have all proper documentation, you can still get rejected without (what I would consider) a proper reason. I won't go into detail on how this impacted my friend but always make sure you have a plan B in place, and think twice before betting on being allowed into the US.