Yes constructive criticism isn't punishable, but what allot of people fail to realize is the "constructive" part of "constructive criticism" is the key thing there. You aren't giving CnC if you're just yelling, raging and flaming someone-you're just being a jerk.
For example: "stop feeding noob" isn't constructive criticism as you're just insultingly telling someone to stop doing something without suggesting a way that they can stop doing it.
On the other hand, "hey GP, try hugging your tower more often and wait for ganks before engaging to avoid dying as often" is helpful advice as it not only gives a suggestion of how they can improve their performance but it also doesn't insult them while doing so.
If you want to give a friendly tip or advice to someone then do it in a mature and respectful manner, and if they aren't receptive of it then just drop it and move on rather than continuously nag and pester them about it throughout the whole entire game.
NOTE: When I say "you" here I mean it in the general sense.