In the case of twitter, the author is being a little bit short sighted. More and more companies are using twitter to form tighter relationships with their customers. It is a no-brainer for Twitter to charge them once it becomes common practice. That's just one example. I predict that Twitter will become one of the net's great success stories and people will have a hard time remembering that when it first launched, it was nothing more than a couple of hundred people twittering "I'm bored."
As far as Seesmic goes, well, it's a little tougher to guess, but my guess is that they're hoping that their video commenting service becomes so addictive and important to companies that they'll pay to have video comments included on their sites. Will they succeed? Unclear. But it's not like when asked "what's your business model" that they said "uh, we don't have one" and it's short sighted to think that. Seesmic also owns that twitter client. Downloadable clients are trojan horses. Once you get enough people using them, almost anything is possible.