Korean stars that dont speak Japanese

Just been wondering about stars such as Park Yong Ha (パク・ヨンハ). He's the only person that comes to mind at the moment, but there might be others.

Park Yong Ha is a singer in Japan, with songs such as "Birds in your cage". He is Korean and sings in Japanese. Something that is quite usual in Japan, other Korean stars such as BoA and K sing in Japanese. However, recently I saw Park Yong Ha on the "HEY! HEY! HEY! Music Champ" TV Programme, and was a bit shocked to discover he couldnt speak Japanese. Dont get me wrong - you dont have to speak Japanese, but... its just... he sings in Japanese.. Doesnt he understand what he is singing about then? Is he just a front man to sing somebody elses songs? It seems odd to my mind, dont Japanese people think the same?