Pirating iPhone apps is easy, and Apple's signature checking that locks down iOS doesn't actually prevent piracy. A pirated iOS app passes the signature check same as the legitimately bought version - it's the same .ipa file, after all. You don't need to jailbreak your iPhone in order to install apps you haven't paid for.
The signature check makes sure 'the developer has paid Apple', and not 'the user has paid for this app'.
Sideloading of apps allows for apps that Google may not care for, and that Apple actively prohibits (eg bitcoin), the difference being that you can still run them on Android if you really want to.
This is incorrect. Unmodified App Store .ipas are DRMed (app __TEXT is encrypted); the signature is of the encrypted binary, and the keys are tied to your iTunes Store credentials on the device. Piracy thus requires someone to decrypt the binary with a jailbroken phone, followed by either:
(1) installing the resulting unsigned app on a jailbroken phone, or
(2) re-signing it with a developer or enterprise distribution key, which Apple can revoke.
Both of these are done regularly, but it's not like Apple hasn't tried to stop it.
By the way, Apple no longer forbids Bitcoin apps, although this doesn't defeat the general point about forbidden categories.
The signature check makes sure 'the developer has paid Apple', and not 'the user has paid for this app'.
Sideloading of apps allows for apps that Google may not care for, and that Apple actively prohibits (eg bitcoin), the difference being that you can still run them on Android if you really want to.