Clarification on a concept in the mnist_basics chapter

Thank you in advance for your help.
Going through the chapter I came upon the concept of, how we are going to check if our model is detecting a 3 or 7. We agree on a threshold on >0. I am trying to find the motivation behind picking 0. Is it because our weights are selected from a normal distribution and presume 0 is going to the average response from the model? I have attached image of the concept from the chapter for clarity’s sake.

I appreciate your assistance.