This is the reason why Microsoft and Google have both been investigated for anti-competitive behavior even though neither one of them was a monopoly.

The Sherman Act prohibits using a dominant position in one area as leverage to exclude competitors in the same or different area.

In other words, a dominant player has to play (more than) fair in any new markets.

I say "more than" because if Amazon was not so dominate in ecommerce this behavior would be perfectly fine.

An antitrust "monopoly" is defined by market/pricing power. I wouldn't be all that confident that Amazon's online retail operation wouldn't be found to have such power.

Using one's market dominance in one field in order to defeat competitors in another field is also sometimes considered anti-competitive, though of course subject to a lot of considerations (e.g., how big your dominance is and what the behavior is specifically).

This is precisely how MS got nailed in the 90s - they were using their dominance of OSes to spread IE and prevent the spread of competing browsers (well, Netscape at the time).

Amazon isn't quite a monopoly like MS was in the 90s, but they're large enough that maybe it matters.

Generally speaking, you are being anticompetitive if you're strong-arming the competition out of the market as opposed to challenging them based on the relative quality of your product. But at what point exactly it becomes illegal I wouldn't know.