Apple does, in fact, prevent you from installing a third-party messenger on iOS that uses the device's native SMS/MMS capabilities. Apple's own Messages app is the only app that is allowed by Apple to handle SMS/MMS on iOS.
The fact that Apple has made iMessage exclusive to Messages, paired with the Messages app's privileged position as the only SMS/MMS client Apple allows on iOS, gives the Messages app and iMessage an unfair advantage over competing messengers and messaging protocols.