No valid reason for not having a Sandbox mode
"We don't want players to grind"
This is just so ironic it hurts. It doesn't even make sense. At all.
Grinding = Tedious
Tedious = Having to go and hunt AI champions and minions for at least 20 solid minutes to try something out.
We are asking you for SOMETHING to STOP GRINDING because we DON'T LIKE GRINDING. YOU want us to GRIND because you DON'T want us to "GRIND".
Not to mention it is COMPLETELY OPTIONAL. You aren't FORCING us to "GRIND" it.
There is absolutely no reason for there not to be a sandbox mode, especially in a game that has pretty big learning curve and a VERY basic tutorial.
What is the real reason you wont add it Riot? I am going to take a guess that you simply don't want to put work into something to please the community and greatly improve your player number and happiness.