Lets look at how baseball works with young talent... Does anyone really think that every single 17 year old who has a rocket for an arm will automatically turn into the next Roger Clemens? No! Teams know that there's about a 5-10% chance that such a talent will eventually become an all-star level starting pitcher but that's the gamble they have to play. They pay 1-3 million signing bonuses for someone who "projects" to be great, but they won't really know for 5-7 years. Once signed, these 17-18 year olds are technically professional baseball players but are they ready to take on major league hitting? Of course not! They need to develop!
So, with so much risk, what's in it for the baseball team (or Adidas, in the case of Drew Hunter)? If (and that's a sizable if) they do turn into great players (or runner) that 2 million dollar signing bonus (or 10 year contract) is chump change compared to what he'll be worth as a dominant starting pitcher (or becomes the next great American distance runner).
TL;DR
(1) There's high risk in signing someone that young/early but also potential for very high reward.
(2) Talent development is a long-term approach.
(3) Just because you're a "professional" does not automatically mean you have to be world class the instant the ink dries on your contract.