I can confirm that this isn't an ad, and that we have absolutely no plans to introduce advertising to the desktop.
Technical explanation
When pre-installing a few of the apps in the Pro edition, their menu launchers (provided by the upstream package maintainers) can sometimes be configured by default to appear in a few menu categories at one time – such as in the Graphics and Science categories for FreeCAD. Not only can this be confusing to some users, it can cause the menu to display some categories with only 1 item in them – like FreeCAD being the only item in the Science category in Zorin OS Pro. This is messy and less than ideal.
As a result, we substitute the menu launchers (.desktop files) of a small number of apps in Zorin OS Pro. They're based on the original menu launchers provided by the package maintainers, but are slightly modified to behave more sensibly. They don't overwrite the original menu launchers, but are new .desktop files that are placed in the /usr/share/[session-name]/applications directory, so they take precedence over the original ones in the Zorin Deskop sessions.
These substituted launchers get removed if the user selects to install Zorin OS Pro with the "Minimal installation" option (which uninstalls the additional productivity apps and creative suite). However, if the user installs Zorin OS with the "Normal installation" option but later uninstalls some of these apps manually, it may still leave behind these substituted launchers.
How to remove the launcher
If you wish to remove only the substituted launcher for FreeCAD, simply open the Terminal (Ctrl+Alt+T) and enter these 3 commands:
sudo rm /usr/share/zorin/applications/org.freecadweb.FreeCAD.desktop
sudo rm /usr/share/zorin-wayland/applications/org.freecadweb.FreeCAD.desktop
sudo rm /usr/share/zorin-os-lite/applications/org.freecadweb.FreeCAD.desktop
If you manually removed all of the additional apps in Zorin OS Pro but still see some of their launchers in the menu, you can remove all of the substituted launchers by entering this command in the Terminal:
sudo apt remove zorin-os-pro-productivity-apps zorin-os-pro-creative-suite