Hockey: the NHL draft is different

Yesterday's post, with an update at the bottom...

Four Harvard Freshmen Selected in NHL Draft

"Months before they’ll put on a Harvard uniform for the first time, four incoming Crimson freshmen were chosen in Saturday’s National Hockey League draft.

With these four additions, there will be eight NHL draftees on Harvard’s roster going into the 2011-12 season.

"The structure of the NHL draft differs from that of the other three major American sports. Unlike in MLB, the NBA, and the NFL, players selected by NHL teams can continue to compete on the amateur level while remaining the protected picks of the team that originally selected them.

Baseball, football, and basketball prospects are forced to choose between signing a professional contract or retaining amateur status and NCAA eligibility shortly after the draft."
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Can someone fill us in  on why the NHL works this way? i.e. why do pro hockey draftees include students who are about to go to college?
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Update (Friday, July 29):

Jaron Cordero writes with some relevant detail:

"NFL draft: to be eligible players must be out of high school for at least three years.

NBA draft: you have to be 19 years old to be eligible.

--So a student can't enter either draft before entering college.

NHL and MLB: you can be "drafted" and still retain NCAA eligibility. In fact, there are plenty cases each year where a player fresh out of high school will get drafted by a major league baseball franchise, but instead choose to play college baseball.

The difference between the NHL and MLB is their respective collective bargaining agreements:

The MLB's requires a team to sign their drafted player in order to retain exclusive right of negotiation for his services. NCAA legislation states that an athlete's amateur status is forfeited if he/she signs a contract with a professional team.

On the other hand, the NHL's CBA allows teams to retain the exclusive right of negotiation of a drafted player until the summer after the athlete graduates from college. Thus, the athlete is not forced to sign any contract with a professional team; therefore he keeps his status as an amateur."

Thanks, Jaron.

So...now I'm puzzling over a new set of questions, e.g. why are the agreements so different? E.g. in MLB, they seem to think that playing in the minors is the way they want to develop players, in contrast to football, where players often develop in college. (Maybe because for football you have to see how big they are going to be when full grown?)  Is hockey somewhere in between?