What is the point of restricting rune editing pre lvl 10?

for an old player like you , it might feel annoying but this is usually for new players

Having achievements and new stuff unlocked each time you level up is what makes games interesting , League isn't the only game that does that

I’d guess that it’s to reduce complexity for new players. It might also be a counter measure to smurfs, discouraging competitve players from easily jumping into games.

stopping noobs from crippling themselves.

To piss smurfs off.