That particular phrase may not be common, but over the years I've come across plenty of free software proponents who absolutely do believe that proprietary software is immoral.
This belief is usually based on the imbalance between the developers and users, such that the developer is able to change the software in unexpected ways, backdoor the user machine via rootkit DRM, spy on users and collect telemetry without the users being aware of what exactly is being collected etc. In that sense, the sentiment isn't wrong.
This does not mean all proprietary software is like this, but the developer/user power imbalance is present in all proprietary software to a much greater extent than free* software.
*Free does not mean non-commercial, it's cool to want to get paid for software, including free software.
A more accurate term then would've been "potentially-immoral proprietary software". Though it doesn't roll off the tongue quite as well and its trolling factor is laughable, so I can see why it wasn't employed.
In my experience it's generally based on the Stallman concept of software freedom, in which the freedoms Free/Libre software is intended to respect are seen as rights, and the power that a proprietary software vendor wields over users is fundamentally unjust. This is the basic philosophy on which the Free/Libre software movement was founded, so it's not just a peripheral view. He outlines this pretty clearly here:
