We've asked both Facebook and Google for their stances on the app and will let you know what they say. Whatever their reactions, though, it's not surprising why Facebook would push so hard. The more you use Messenger for your chats, the more likely it is that you'll use Messenger for purchases and other services that might help Facebook's bottom line. It's just a question of whether or not the social network is being completely honest with users who don't realize that SMS integration is strictly optional.
Update: In a statement, Facebook tells us that it's still being fair. You go to "settings" (as expected) to decide against using Messenger for SMS, and you'll get a permissions request before the change happens. That's important, but the concern is more that Facebook doesn't have a conspicuous "no" or "skip" option that makes it clear this integration isn't mandatory. You can read the full statement below.
"SMS in Messenger is an optional feature. People can choose whether or not they wish to use it. When they first see the prompt, they can choose to start seeing their SMS messages in Messenger by turning on the feature, or they can decide not to by tapping "Settings." If they decide to see SMS messages in Messenger and to also reply to messages from Messenger, we'll ask people to approve any new device permissions that are required. Messenger doesn't modify any device settings without people agreeing to it."