He's a fantastic prospect - one of the best I've seen - but no one knows the future. Some pitching phenoms don't deliver as expected (see Chamberlain, Joba). Others get hurt (Prior, Mark). Moore's left arm could fall off tomorrow - he hasn't had TJ yet - and in a few years he'll still be a 27 year old with $7M or so in the bank. If he plays out the entire contract he'll be a 30 year old free agent with $20M plus already secured. He'd still be well positioned for a huge pay day.
No one made the kid select a particular agent or sign this contract. After just one month in the big leagues he's basically set for life. Other players take the risk and sign short contracts. Papelbon made it clear several years ago that he'd take his chances because he wanted to be a free agent at the first available opportunity. It paid off for him. On the other hand, the year to year approach wasn't quite as successful for Chien Ming Wang (one bad step ruined a very promising career).
Edit: I just looked at Cole Hamels' history. In his third year, coming off a 15-5 record, the Phillies paid him just $500K. It was only entering his fourth year, his first eligible for arbitration, that he received a 3 year $20M contract. That was after winning the World Series and NLCS MVP awards. In his case I somewhat understand the reticence to offer anything long term earlier because I believe he had some injuries in the minors, but it goes to show that even wealthy teams utilize their CBA leverage and generally don't do what the Rays do - offer long term guaranteed contracts to players without a well-established track record. They are taking a chance but it could pay off big for them. Let's see if the Yankees offer Jesus Montero a 5-8 year deal after the highly hyped prospect had a .996 OPS for his first month in the big leagues.