As the "Project Files" stores the files as they are, and the "Plans" folder processes files by breaking down Sheets, Views, implementing Tileblocks etc., they have different functionalities and uses. The "Plans" folder basically houses the digital equivalent of the printed set of contract drawings for the project. I would imagine this means the public link feature would have different implications in terms of allowing access to an entire file, or only to certain parts of a file.
If you'd like the feature to be implemented for the "Plans" folder as well, you can post this suggestion on the BIM 360 Ideas Forum.
Andrei Nicolae
Technical Support Specialist