If ads are a valid funding model and payment is a valid model, why wouldn't both together be valid? What matters is the total price, now how it is broken down.
Imagine, when buying your groceries, you had to pay half cash, half by prostituting yourself. Nobody would want that.
Though, prostitution is a valid funding model, as sex workers earn a living with it.
To extend the metaphor, I see advertising as some kind of prostitution for my brain. I do not want that for my brain.