It is by design. All those work in the same direction:

    dest = src // Variable assignment.
    <-ch; // Read from channel.
    // in net/http
    type HandlerFunc func(w ResponseWriter, req *Request)
    // in io
    func Copy(dst Writer, src Reader)
    // builtin
    copy(dst, src) // Slices.

So the reason is consistency. I suppose the Pipe exception is to map Unix's pipes.