[Twisted-Python] Interfaces and adapters

Let's say we defined an interface and an adapter:

class IFoo(zope.interfaces.Interface): pass

twisted.python.components.registerAdapter(Foo, Bar, IFoo)

As it is for now, the registerAdapter accepts any Foo, without regarding what it implements.

Could somebody give any rationale why this condition shouldn't always hold:

foo = IFoo(bar) assert IFoo.providedBy(foo)

Seems it is not unreasonable to expect this behaviour, because it is exactly what interfaces are all about, or am I wrong?