When you change channels, you're no longer consuming the content on that channel. It's closer to using a TiVo to skip the ads in a programming. I do it myself, but it's pretty obvious to me that if everyone did it, it wouldn't scale and they'd find some other way to create revenue like more product placements during the show (which is already happening) that's even more intrusive and obnoxious, because it's unlikely that providers or consumers are going to go a-la-carte.

(This is also why I liked paying for content on iTunes or Amazon - that seems like a more fair deal where I don't get both charged and ads (cable) and I don't have to mess around with using the TiVo to skip ads.)