Update: One June 21 Facebook reached out with the following statement on this matter. "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." End of update.
Facebook recently gave its Messenger app the ability to handle all of your SMS messages. The app brings the same rich messaging experience to SMS messages that users currently enjoy via the app over a data connection, such as emoji and stickers. While the SMS and MMS formats themselves are limited, Facebook Messenger works nicely within those limits to get SMS messaging as close as possible to the data-hogging real deal. Some users who have Messenger, though, want it to keep to Facebook messages only. Such users may have found it a bit hard to tell the app to do just that, and it's this overly aggressive behavior that can fool some into thinking that SMS integration is not optional and, in essence, they must have Facebook Messenger as their SMS client in order to use the app at all. Slightly less knowledgeable users may simply think it's another splash screen they have to click through to use the app and have no idea that they just disabled their favorite SMS app, which might be against Google's rules for apps in the Play Store, despite the explicit user involvement.
While the app only gives the appearance that SMS is mandatory, there are rules in Google Play's policies against apps changing users' device settings against their will or without their knowledge. Naturally, which app is currently your SMS client is a device setting. In Facebook Messenger, users are presented with a splash page upon launching the app that tells them about the new feature, and has two buttons; OK, and settings. The OK button will take you through the process of enabling SMS, which will disable any other SMS client on your device and force you to use Facebook Messenger for SMS. The Settings option takes you to settings, and from there, you can back out of the SMS option.
While the option being set to yes for users is clearly an app setting, users that don't know any better may simply hit OK and go through the prompts, then wonder why their favorite SMS app is suddenly not usable. Other users may know what's going on and accept it as the price to pay for chatting with their friends on Facebook. While both scenarios are a bit on the deceptive side, only the first is actually runs a risk of running afoul of Google's rules; since users may accept the SMS setting without knowing the full story, these users' settings are technically changed without their knowledge. For now, neither Facebook nor Google has said anything about this "feature" of the app.