OK, so "preferences over the total amounts of money donated to each charity" mean that you ignore any information you can glean from knowing that "the other guy reallocates to $0/$1000", right? Like betting against the market by periodically re-balancing your portfolio mix? Or donating to a less-successful political party when the balance of power shifts away from your liking? If so, how does it imply that "your participation in politics should be 100% extremist"?
you ignore any information you can glean from knowing that "the other guy reallocates to $0/$1000"
Good point, but if your utility function over possible worlds is allowed to depend on the total sums donated to each charity and additionally on some aggregate information about other people's decisions ("the market" or "balance of power"), I think the argument still goes through, as long as the number of people is large enough that your aggregate information can't be perceptibly influenced by a single person's decision.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.