Prisoners are being punished
Respectfully, I must disagree with Patrick Condon. People in jail are awaiting trial. 2020 saw some terrible postponements of fair trials due to COVID-19 but that is not happening now. They are likely criminals, and getting mail could potentially include things not meant to be mailed. Committing a crime does not give you the right to have the same privileges as law abiding citizens.
I worked at a book store and you can not buy a book and send to a jail or prison. It has to come directly from the publisher. Not getting mail is not the same as being denied food, or the ability to have hygiene.