@Itchthescratch the cheaper days are literally paid by charity grant funding. It is not impacting the price of "normal" tickets why are you still upset that poor people, who are in need of charity are getting it.
I am going to quote myself because you are wilfully ignoring the answer to the question you have asked
"It is a grant funded scheme called Community Connections raised from various means including, the national lottery, local development levys, various other charities, including Age UK etc"
The program aims to make the zoo accessible to low-income families and those with additional needs or disabilities.