> The web has no default client.
Every computer comes with a browser. Back in the dark days, ISPs geared towards new users were full-on content providers ala AOL.
> Email has no default client.
Again, either your ISP sets it up for you, you're using your OS's built-in (Windows Mail, Mail.app) or you're using GMail or similar.
> TV has no default client.
Sure it does, it's baked into every television set ever sold, and it's started when you turn the set on.
> SMS has no default client.
Again, buy a phone and it's got an "SMS client." Receive SMS, phone alerts you, you can respond.
However, RSS has no default client. If you see a link that says "contact us via email" and you click it, your mail client responds to the mailto: link. There should absolutely be an rss:// (or whatever) protocol link that browsers can either handle internally or ship to whatever client.