I like Allo, but I’ll admit, the way Allo manages messages is a bit confusing. People wanted to make Allo their default text messaging app, and that also adds to the confusion.
Allo Messages Without Installing Allo
A clever way that Google markets Allo to new users is this: if you send messages to people who do not have Allo installed, Google will send the message through Google Play Services (a service that provides required support for all the Google products that you run on Android devices) and ask them if they want to install Allo.
Your friend who received that message has two options. They can either install the app and continue to chat, or continue to talk to you via text, for free. The number from which they receive the message will not be users, a 5 digit number instead.
Oh, there is a third option as well, they can ignore your message!
The Free SMS
The free SMS deal happens when your friend replies to your message, opting to communicate via SMS.
Since this is normal text message, you can receive, only, ahem, text from your friend. No emojis, photos or stickers here.
Stop Receiving SMS From Allo Users
If you don’t want anything to do with Allo, and want to stop text messages from Allo , text “STOP” as reply to one of the messages and that should take care of it.
If you are looking for some official documentation on how this feature works, here is the Help Center article from Google.