"Don't play ranked until you practice in sandbox."
Why would they (Riot) discourage actual gameplay against actual opponents trying to win? They don't want anyone to feel pressured or to depend on a sandbox mode to be good; which, without a doubt, is the inevitable fate of this practice field.
The rage towards players having a bad game would increase dramatically. If you can't see that, then kindly remove your blindfold. 
There's a large enough flaming issue as it is, and all this community does is throw twigs at the fire.
Riots logic here is both understandable and reasonable.