Amazon has an audiobook monopoly and takes 87% cut of audible sales unless the author agrees to exclusively sell through audible. If you agree to exclusively sell through audible, Amazon only takes 79% cut of the sale. Officially they claim to take 75% and 60% cuts (for non-exclusive and exclusive), but they actually pay out considerably less than they promise.

That's what a monopoly abusing it's power to steal from creators looks like. Valve's 30% is literally market standard, and is so much lower than they could get away with.

Sources on how much audible takes per sale: My source, Original source