why some video music content in yahoo player says only available at certain areas?

Answer 1

The rights-holder of the video may license Yahoo to play the video only in certain countries. If Yahoo determines that you live in a country where Yahoo's license is not valid, you will see the error message you mentioned. Other videos may have a worldwide license, so you won't see the error on every video.

It's unfortunate, but that's the way it works. Blame the record companies.