The fact that you do not assign an action to a button is seen as a choice by the Mendix Studio, and therefore shown as a warning. You can (and I'm happy) actually deploy an application to production having no associated event bound to a button. I think it's a feature.
Hopefully this helps!
So do older versions, 7 and even 6:
The “feature” for this is having a placeholder for showing an icon:
So, you are correct in expecting this to be an error, but in Mendix it is probably seen as a feature.
And yes, this will do no harm to your application when running in production.