I don't think this is necessarily true. The proposal was that the cost be
negligible for "normal" emails and it's only when one tries to send
hundreds of thousands out that it becomes "real money". It would not
affect the average person at all.
I don't see any way to impose costs that are sufficiently large that
they will deter spam, that aren't so large that they will chill valuable
speech.
I do admit that such costs provide an incentive for spammers to target their
mail, which would probably reduce the volume from any one spammer, and it
would compel them to implement opt-out schemes. But there would be a lot more
spammers.
As for challenge/response schemes, they're fine for people who want to use
them. But I find them so annoying (even with whitelists) that I'm unlikely to
impose them on people who send me mail. Maybe if we could automate them
somehow so that users didn't have to deal with them directly, they wouldn't be
so annoying.
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