Why is CNBLUE accused of copyright infringement?

Mnet made an official announcement on a copyright suit between Crying Nut and CNBLUE.
On February 13, a spokesperson for Mnet said, “We admit that it was our fault. We will do our best to help the two groups come to an amicable agreement.”
Crying Nut’s agency, Drug Records, filed a copyright suit to the Seoul Central District Court on February 12 against CNBLUE’s agency, FNC Entertainment, claiming forty million won in damages.
According to Crying Nut, CNBLUE used a sound source without permission and released their performance as a DVD in Japan.
The performance was given on the June 17 episode of Mnet’s M Countdown in 2010. On the show, CNBLUE performed Crying Nut’s world cup fight song “Deadly Offside.” And the video of their performance was included in CNBLUE’s special DVD released in Japan in August, 2010.
A spokesperson for CNBLUE said, “CNBLUE had to give a sudden performance with the sound source provided by Mnet. We didn’t know that the video was included in the DVD. We didn’t even make any profits from the song.”
Mnet said, “We gave Crying Nut a monetary compensation last August. I think they field a suit against CNBLUE because they performed the song. We made a mistake and all problems have occurred because of us. We will try hard to iron out all the mutual misunderstandings.”

• Source: TV Report