My thinking on why Riot doesnt want sandbox mode.

I think that riot is most doing this so that in ranked, everyone isn't expected to be perfect at everything in even low bronze. If this were to exist, instead of "LOL THAT FLASH" it would be more along the lines of "why havent you spent a large amount of time making sure that you don't miss that you noob." This wouldn't be TOO bad if it would likely move into normals, where people play to have fun. On the other hand i do support a better and more all encompassing tutorial, which isn't the same but deals with another problematic area.

Basically; Riot doesn't want perfection to be expected, which it would with a sandbox mode