IDE won't open the .cpp file.
I wouldn't to put words in anyone's mouth, but I think the Arduino developers may have reasoned that people who want to look at .cpp (library) files would use a "real" programmers text editor rather than the toy editor built into the iDE. (So you can't open just any old .cpp file in any old directory).
You can still create tabs in your own sketch and name them something.cpp or something.h and Arduino will create the files and save them in the sketch directory, but I think the IDE only lets you open "sketches" (by navigating to a directory that contains a .pde file) in the IDE File menu).
You can also create .cpp and .h files in a sketch directory with an external editor.
Anyhow...
Once you open a sketch, then the .pde and the .cpp and .h files in that directory appear on tabs in the IDE. Since a library directory is not a sketch (has no .pde file), I don't think you can open a library .cpp directly in the IDE. (I could be wrong.)
I've not been around long enough to know anything about releases older than about six or seven months, so I don't know if or when the functionality changed.
Regards,
Dave