That makes some, but not much sense to me. Firstly, that is not what I read in the book "The Swift Programming Language":

"Whenever you assign a Dictionary instance to a constant or variable, or pass a Dictionary instance as an argument to a function or method call, the dictionary is copied at the point that the assignment or call takes place."

Even if I assume that that is an error (they didn't think of inout arguments when writing that), the behavior still doesn't make much sense to me.

Say that I want to do a few things with someObject.someField.someDict. In most languages, I would introduce a helper variable: var items = someObject.someField.someDict and do items[42] = 346; items[423] = 356; etc. That won't work, as it clones the dictionary (shallowly). I find it is just too easy to accidentally clone arrays or dictionaries in Swift.