That's a terrible analogy. Email is paid by each end of the connection paying their ISP for the network and possibly mail server access, and the sender being willing to do the work of creating the email for free (or getting paid through an outside mechanism). Contrast that with youtube where the provider of the video needs to make some money to keep their lights on and pay for their end of the network traffic. Either you have ads to pay for the other end of that network connection, hosting fees, and the content creator, or you need to workout a subscription or micropayment system.
Besides, SPAM works completely outside of the email payment system by frequently hijacking a host with malware and use their resources to send the junk. Ads, on the other hand, pay the person that is providing the service or content, and are only pushed to you when you request the content. Don't want to see an ad? Then don't go to an ad supported website. Don't want to see SPAM? Sorry, there's no easy opt out for that.