In a home it is a wanted feature, but in a business place with lot of people, it is dangerous that anyone is able to control if a sound application will start to reproduce media. The multimedia support in the lock screen should be configurable, it is shown (default) or not.
A possible dublicate of https://bugs.kde.org/show_bug.cgi?id=384264
*** This bug has been marked as a duplicate of bug 384264 ***