Not a lawyer, but in my understanding, the core property of a contract is that both sides are aware of it, in particular of their obligations in the contract. There must also be a defined moment the contract is concluded.
This is specifically not the case with ads: Ad-supported services are frequently advertised as "for free", not in the sense that ads are the "payment". Even if they were, they would be unlike any other business transaction as the service provider is free to change the "price" (i.e. amount of ads shown) at any time.
That's not even considering all the situations where you're subjected to ads without receiving any kind of service - or where something that you paid money for suddenly starts to show you ads too.
Felony contempt of business model indeed, as well as theft of assumed future profits!