I think it's for the streamers and youtube commentaries. If you can live spectate a game with pause and rewind, it's more than possible for them to put that in a file and let you save it like sc2. But there will be people like me, and other who would prefer to watch a replay over watching a dude's perspective of the game. Not only does it give those streamers and youtube commentators more money and a reason to keep doing what they're doing. It also generates more word of mouth advertising in the form of a lot more replays on youtube and commentators advertising for themselves.
Riot wouldn't make money on giving you a replay, they would make money if more people were into lol. It's not a bad thing in my eyes, it's just the way it is.
Or they could be too lazy to implement the function, but I doubt that's a random choice, just my 2 cents.
Yeah so we are kind of alpha testing it right now on PBE!
We do things at Riot because they are cool, they make our game ever more awesome every day to serve our players. Streamers are players, too.