It doesn't take 2 weeks to be good enough to play a normals game with a champion. It also doesn't make much sense. I could go 1st time Vayne in normals/ranked (which I actually did due to a trade not going through :C) and I'm not going to know how to play her, hell I might not even own her and it might just be Vayne free week. I'm gonna be quite nub with Vayne even though Vayne has been out for a long time, and you deal with that in all sorts of games. It's the same with Kalista. Normals is to try out new (and sometimes free week) champions. You're going to not be pro at any champ the first time you play them and restricting a champion to bots for the first 2 weeks means that bot games will be unenjoyable (everybody playing bots will just try to instalock the new champ and everyone else will dodge) and that sales for the champion would be a lot lower because nobody is going to pay a higher price tag for a champion they won't play for 2 weeks anyway.