Regarding the thought that Instapaper is somehow robbing writers of ad revenue, I think it needs to be noted that you actually have to visit an article to add it to your Instapaper queue. On top of that, Readability's model of paying authors isn't all that effective, a recent article on The Brooks Review[1] showed that even a moderately popular site such as his generates nearly no income from Readability's service, especially now that the service has gone free with optional payments.
Also, what happens to all those authors that don't opt in to receive payments from Readability? Does readability just take money on their behalf and not actually pay it out to them? It seems like that would be a legal grey area.