Have you already read Charles Stross's FAQ: Why is there no tipjar? (http://www.antipope.org/charlie/blog-static/2009/03/reminder-why-theres-no-tipjar.html) The key paragraph:
"If I [Stross] put a Paypal tipjar on this blog, to take conscience money from folks who've downloaded a (cough) unauthorized ebook or two, the money would come to me, not to the publisher. And without the publisher those books wouldn't exist: wouldn't have been commissioned, wouldn't have been edited, wouldn't have been corrected and marketed and sold in whatever form filtered onto the unauthorized ebook market."
Many authors regard their publishers as partners, and don't want to take revenue from them.
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"If I [Stross] put a Paypal tipjar on this blog, to take conscience money from folks who've downloaded a (cough) unauthorized ebook or two, the money would come to me, not to the publisher. And without the publisher those books wouldn't exist: wouldn't have been commissioned, wouldn't have been edited, wouldn't have been corrected and marketed and sold in whatever form filtered onto the unauthorized ebook market."
Many authors regard their publishers as partners, and don't want to take revenue from them.