Money grab. The game will constantly remind you how you can spend real money in order to progress. Virtual coins everywhere, forget the "beat this track in less than X time to unlock the next bike", everything has a price tag, and a rather expensive one. Either waste an insane amount of time grinding, or start spending a lot of money. Nevermind the "... game doesn't start when you get bike 7..." response to most reviews trying to sugarcoat the paywall that's around every aspect of the game.