Excellent examples. I suppose that I'll amend my statement to say that sarcasm is not ~necessarily~ conveyed through word choice, and often relies on vocal inflection (especially in casual, personal conversation).

I understand the value of indicating sarcasm in a text message, because sarcasm is not conveyed through words, but through vocal inflection.

That being said, I don't advocate any person or corporation making a business of this limitation. As thousands of internet forum-goers already do, you can end a sarcastic remark with [/sarcasm] or some other such indication.