why do I need wifi to listen to my Audible book purchases on my new Ipod Touch

I have a new Ipod Touch (7th generation) and download audiobooks from Audible. I know I can listen to these books when I have wifi but I like to listen to my audiobooks when I walk outside. I have downloaded them to my hard drive of my MacBook Pro and then use Itunes to drag them onto my Ipod. They will play for a few minutes and then I get an error message saying that I need to connect to WIFI. How do I download them completely to my Ipod Touch that I purchased with 128 GB? Thank you!