It is too expensive. 1TB of bandwidth costs about $120. A project like adblock plus will be consuming about 3 - 4 TB a month which will add up to around $450 a month.

Adblock list subscriptions are maintained and hosted by individual people who do at their spare time. They mostly pay for the servers out of their pockets. As one of the co-author of popular adblock list, I wouldn't want to break my bank to pay for S3 hosting. Our current solutions works out and when we reach our bandwidth limit, we could just simply buy addition TB of bandwidth at a much cheaper price than S3.

Btw, i just made a rough calculation using AWS simple monthly calculator. So correct me if I am wrong about S3 pricing.

While this workaround has merit, it doesn't actually solve the underlying problem. I guess even Amazon will eventually pick up the phone and ask you to stop sending them weekly bandwidth spikes when the figures involved get large enough (I've personally seen this with another well known PaaS provider).