Yasuo is actually a doctor

He's well aware of the wisdom of whoever bans him, saying 'no cure for fools' Of course he's referring to the fact there is no way to cure certain illnesses other than through prevention, which is a cure not for fools.

Yasuo is well aware of the importance of prevention over treatment, and is reminding us to take caution in our health and lifestyle. So remember to put on sunscreen and avoid eating unhealthy food.