Apple's App Store insider knowledge lets them discover popular apps, then boot them off the platform when they decide to compete. Amazon sees what products are successful and profitable, then makes knock-offs which they promote over the originals. And now, reading into this, Google is manipulating their ad markets using inside knowledge.
It's all very profitable.
If we cannot trust large companies to not abuse this kind of power, then they should be prevented by law from playing both sides. This may result in the breakup of some very large companies, akin to the 1934 Air Mail Act that broke up Boeing.