I don't understand why should String Theory be labeled as "scientific". Can you explain? Why cannot we be happy just calling it philosophy?

To me, "scientific" means "uses scientific method", which in turn is a set of tools that are pretty successful in understanding universe. Scientific method is certainly not fixed set of tools - for example, computer simulation became very useful only recently.

Maybe doing whatever string theorists are doing will become useful in the future, and as such their method will become part of scientific method; but since we aren't really sure if string theory itself is true, we can't consider their methodology to be useful (or useless) and so part of scientific method.