It appears, according to this analysis, that Amazon is not gaming the system in favor of their own imprints. However, some genres are far more competitive than others and may be dominated by either the Big 5 or Amazon’s imprints.
From the article:
“…there is no sign that Amazon has its finger on the scales. I have seen no evidence of algorithmic manipulation of Amazon’s search results, sales rankings, or reviews to favor their imprints. In the short term, Occam’s razor applies: the rise of Amazon’s imprints may simply be due to a potent combination of quality books and savvy marketing.”