Why does "Sore Ga Seiyuu" play other anime's OP/ED as its ending song?


I found that when I watched _Sore ga Seiyuu_ anime, the ED songs seems from
the other anime. For example, in ep. 1, they sang _Evangelion_ OP, and in ep.
2, they sang _Sayonara Zetsubou Sensei_ OP.

Here are some examples.

  1. [Evangelion OP ( Ep. 1 )](https://youtu.be/z6G_16zh510?t=23m19s)
  2. [Sayonara Zetsubou Sensei OP ( Ep. 2 )](https://youtu.be/HykGVek1rzU?t=23m18s)

What's the reason they are doing that?

