Itunes BPM detection

You use a third party tool or source to generate the BPM data. Why they don't have it you would have to ask Apple but I could easily see it simply being regarded as a separate feature that isn't necessary to the functioning of iTunes and quite frankly may not really be needed by many users (I don't use BPM). iTunes is really a standard consumer level product, not a professional DJ tool though you can use it that way I guess.

How can a DJ use this for a library with no BPM auto detect ?

Apply a 3rd party BPM detection tool such as Beatunes or Mixmeister, and store the BPM in the appropriate field in iTunes.