I wrote the previous post at home without having the benefit of having the Library of Congress Subject Headings (LCSH) volume with me. I've just checked to see if my recollection of LCSH was correct, and as a result, I need to make a minor clarification. In LCSH, miscegenation is preferred over equivalent terms such as "hybridity of races", "racial amalgamation", and "racial crossing". More importantly, it says that if one is using "Miscegenation -- Law and Legislation", it is actually better to use "Interracial Marriage -- Law and Legislation" instead. So it's quite possible that the book in my anecdote could have been catalogued as "Interracial Marriage -- Law and Legislation" and still been fully compliant with LCSH.
Still, my main concern remains valid. Miscegenation should be less preferred in all instances, not just for "-- Law and Legislation". The presence of the exception shows that LCSH is aware of the problem with using miscegenation. Now they need to implement a full replacement - or replacements - for the term.