Firstly the C++ Language won't allow pass by value because the copy constructor is private for streams. So why does C++ choose to implement streams this way? Its pretty simple when you understand how operating systems assign file handles to running processes and what the scope of those file handles are(streams eventually work there way back to these primitive constructs). When a process starts executing, the operating system will create a number of file handles(streams) automatically, the std::cout, std::cin, std::cerr, std::clog which are global to the running process...By global I mean the entire process has access to them. The same rules apply for any stream that the process opens. So what does this mean? Well if you think about it, it makes sense to make the stream's copy constructor private. What would happen if it wasn't..Lets try an example using pseudo code.

Create file stream fs.
pass fs to function by value.
fs is copied by copy constructor which is public for our example
function does something with copied stream
function exits destroying fs with fs's destructor
..
Now what happens to our original fs? Isn't it now destroyed since the copied fs freed its resources?