Is it ethical to bribe students? Are some bribes more ethical than others?

These questions crossed my mind when I read of a university that offers scholarships to students who get in the top 5% of grades in the entrance exams. The publicity says the scholarship is only for students who are serious about studying. Which kind of lets the cat out of the bag as to what they think of the other 95%.

I mean, whatever happened to the idea that learning is its own reward? Why should you have to bribe students to study?