Many of those games are actually implementing gambling with their virtual currency. They have events where you buy an entry ticket using the in-game currency then either double/triple-up or lose-it-all as the result of some random event.

I'm surprised this is even legal in the US, and not caught for app store TOS violations. It seems they've simply replaced the concept of casino chips with virtual currency. They do have some 'gameplay' outside of this but it is not much different from the various frills on video slot machines.