Site features (here
site means "site collection", while
web means root-site/sub-site) can only be activated at a site-collection level (i.e. from the root-site of a collection, under the "Site collection features" page).
Site features produce their effects on the entire site collection. For instance:
- If it contains a custom action, the custom action is visible in all sub-sites.
- If it contains a Web part, the Web part is provisioned in the site collection Web part galery (located at the root-level) and is then available for all sites in the collection.
Some artifacts cannot be scoped
site (see list at msdn).
So, either your feature is
site scoped, or it's
web scoped. There's no point at all in trying to activate a
site feature at a
web level. And it's not your responsibility to activate a
site feature on all sub-sites: intrinsically, the
site feature impacts all sites (webs) of the collection.