It's about fun

Riot's stance on sandbox mode is optimizing for a fun, positive player experience integrated over their entire player-base. There are way more casual players than hardcore ones, and from the casual perspective, it is frustrating to lose to someone just because they practiced some combo a million times or are animation cancelling with flash. Yes I might know that I could go into sandbox mode and do the same thing, but also maybe that doesn't sound like fun to a more casual player. The perception (remember, perception) that a player is held back by not practicing these flashy moves could push casual players away from the game. This is a risk Riot does not want to take.

At higher levels of play, yes, I can back it and I understand its importance completely. What people are failing to realize is that Riot is sustained by its entire player-base. Not just the spikes who are trying to eek out an edge.