I guess you're not following Amazon earning reports closely. In Q2 this year, Amazon's margin was number 1,428, out of 1,515 US companies [1] [2]. That's barely 1% after-tax margin. In several quarters they reported close to break-even or losses.

At these levels, a "mere" 0.5% donation would be unsustainable for shareholders, unless there's a way to claim most/all donations to reduce their own tax payments.

Margins got better in Q3, but still (intentionally) low.

[1] http://www.forbes.com/sites/greatspeculations/2013/05/21/ama...

[2] http://247wallst.com/investing/2013/07/25/amazon-com-company...