You might have a point if Chrome hadn't been the first browser to implement pinning, therefore defining the concept in web context to a large extent.

You may argue that this is is broken behaviour, but that's what pinning currently is in browsers. Seems it's this way in Firefox too ("pinning not enforced if the trust anchor is a user inserted CA, default" - https://wiki.mozilla.org/SecurityEngineering/Public_Key_Pinn...)