hedwards: Because you're receiving it whether you use a TV or not. It would be ridiculous to suggest that you're receiving it because you're being hit with the waves all the time, but if you're receiving the broadcast it can be online, via TV or some other method.
SirPrimalform: You're not necessarily receiving it at all. Believe it or not, there are some people who don't watch TV (not even on their computer).
You have pointed out a problem that they don't seem to have worked out yet, I don't think you need a TV licence to view the iPlayer. I imagine they're thinking of ways to implement that right now though.
It doesn't really affect me directly, I just think it's sort of an odd way of doing it. I get taxes for things that everybody uses like NHS, but I think that the American system of just paying for TV if you use it or watching commercials makes a whole lot more sense.