Damaging Yasuo´s shield and not getting an assist/kill. Intended? [COMBAT]

SO, my English sucks but THATS OK. (i hope.) Imagine this: You are playing Syndra. Yasuo is diving..... You only have enough mana for your E. You use E. It hits - and it burns his shield. You will make it out alive. He dies. EXECUTED. Could you explain in more details, what was your...ehm...thinking process behind not giving a kill to someone who deserves it?

I already post this discussion (OR, SHOULD I SAY DISCUS ?? hahahaha that was awkward) in "Gameplay & Balance" section and they were telling many things - some said it´s intentional, that this is how the game should work. Others recommended me to post it here. Thanks!