It's not about that.
I have no issues buying games on GoG, Origin, Uplay or Battle.net
It's about this:
Any type of executable-level modification or injection, as well as changes to data files in some cases.
So, say goodbye to
Performance analysis overlays.
Graphics injectors.
Integration of useful external applications like Mumble.
Community fixes or feature extensions.
Freesync.
In-depth modding.
Support for popular peripherals like the Steam controller. (!)
Some GPU features such as SLI.
Actual exclusive full screen mode.
.