1. Normal people don't have access to HFT equipment, giving a handful of corporations an edge over the rest of us.
2. Most people really don't understand HFT.
I think #2 is actually the real reason people are up in arms about HFT, honestly. It's new, scary, and allows corporations to extract money seemingly from nowhere.
I think the "extract money seemingly from nowhere" might be the operative part here - one thing that almost everyone shares is an instinctive understanding that money does not come from nowhere. In the absence of further information, many will be suspicious that it's probably coming from themselves.
I'm with you on the reasoning, but I hate the logic of
1. I don't understand phenomenon X
Therefore,
2. Phenomenon X must be stopped
To your point #1 in particular, it's true that normal people don't have access to HFT equipment, but they also don't own warehouses full of third-party goods (and would find those probably more difficult to obtain), so it's hard to explain in those terms why Amazon's hypothetical maneuver is different from HFT.
I think #2 is actually the real reason people are up in arms about HFT, honestly. It's new, scary, and allows corporations to extract money seemingly from nowhere.