Because typically the people who buy high end chips have more money than sense.
*shrug* I really don't know. It bothers me when it looks like a company repackages something and raises the cost. As far as I can tell all they did was give it a new ID with a lower standard voltage and add an 'S' to the box. For some reason I don't think that is worth $100.
A Q9650 costs much more than a Q9550 yet they're basically the same chip. They only changed the default clockspeed (multiplier) and ID and then charge an arm and a leg more.
The same thing happens here. Instead of changing the default clockspeed they changed the default voltage. And just like in this case, where anyone can undervolt their chip, anyone can overclock his (cheaper) chip and end up with the same or better performance than the pricier chip.
How is one thing acceptable and the other isn't?
Exactly, it's pretty much all the same thing: they make a batch of chips, determine the performance level of each one, and bin them accordingly. The manufacturer guarantees the performance at stock speed/voltage/core count, and depending on the exact sample you get, you may be able to over-/underclock/-voltage (or unlock extra functional cores, such as some Phenom X3s may have) or you may pretty well be at the sample's limit.
You pay for the performance guarantee and for not having to do the configuration yourself (since it's the default level of the CPU). It's up to each of us to determine whether the extra cost from one product to the next is worth the guarantee or if we'd rather take the chance and try and reach that level with a "lesser" processor.
But, the extra cost for the "S" models is certainly hard to swallow unless you really need that guarantee (can't think of any specific examples, though) or don't have undervolting options in your BIOS (and really needed that handful of watts). Presumably prices will come down if as few people find them "worth it" as it seems like, though with as little difference as the undervolting seems to make here, it'd have to be a fairly small premium over the standard models to make it "worth it" to most of us, I think....