I'm pretty sure that's against Apple's App Store T&C and the application would be banned by flooding fake 5 star reviews. It would also be pretty easy to detect.
Was the app ever banned or everything proceeded as normal? Is Apple in Circle's pocket somehow?
Apple is known to loosen its rules around big apps. Facebook and other big players have gotten away with many privacy violations that any small app would be blacklisted for, simply because Apple knows banning them would damage their brand more than making special rules for big players does.
If Apple were to ban Coinbase, many people would probably blame Apple for restricting access to their cryptocurrency rather than thank them for standing up for fair app store reviews. Instead, they've probably just asked them nicely to stop doing whatever would be ban-worthy otherwise, with a ban (and a press statement) ready in case they don't cooperate.
It wouldn't make business sense to ban a popular app for something as silly as fake reviews, especially with the amount of customer money involved.
Was the app ever banned or everything proceeded as normal? Is Apple in Circle's pocket somehow?