In the slide shown below by red arrow we have quantization noise up to fs/2, but why?

From Nyquist fs=fb/2 for good sampling but why is there noise here noise up to fs/2?

What is the logic ?

[![enter image description here](https://i.stack.imgur.com/nAKWi.png)](https://i.stack.imgur.com/nAKWi.png)