For me, I am convinced that they create jobs, spend money in the economy, help provide valuable goods and services to society, etc. Amazon is famous for not hoarding money but re-investing their profits. Most 401ks invest in index funds that are buoyed by the tech stocks. I don't have ready citations but I believe these to be relatively uncontroversial statements. This whole talk about "net positive" and what is "net" is a pointless discussion that is going nowhere. There is no way to prove anything unless we have an alternative universe without Amazon to study.
What sort of citation will make you happy?