That would be one form of anti-competitive behavior. Using one's market dominance in one field in order to defeat competitors in another field is also sometime

That would be one form of anti-competitive behavior. 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.