I disagree with the premise. You are not paying for content, you are showing appreciation towards the creator (or curator) *after* the consumption of said content. Big & important difference.

I would also like to stress that calling V4V payments "tips" is self-defeating in some sense, as most people think of "tips" as very small payments that are unnecessary/annoying/whatever. "Call it tips and you will receive tips," to quote Adam Curry.