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Webarts cannot be "Web" level features


I agree that webpart deployment is basically a "Module" element and technically can be a web level feature. but *.webpart files are special "Modules" that needs to be uploaded to webpart gallery.
They cannot be WEB LEVEL feature, cause there is no concept of WEB LEVEL WEBPART GALLERY.
so basically you're confusing deployment team by making it a web level feature. turning it on a subsite will do no good and you cannot see the webpart available in "Add New Webpart" window. only if you activate this "web level" feature at rootsite can you see it available for entire site collection.
hence a site collection feature. web feature simply confuses everyone!