I consider it keeping things professional to sit in the back. They are being paid to performing a service for me; it's not like it's slavery. I don't find myself feeling guilty about not standing when a waiter comes to my table.
In the early days, Uber's slogan was “everyone’s private driver” while Lyft was “your friend with a car.” Uber's marketing was aimed at recreating the experience of a private chauffeur (which in certain older times would have been a member of the servant class), while Lyft's marketing was aimed at recreating the experience of carpooling with a friend.
Obviously neither marketing story was really true, but they did influence the way the customer experience was and is presented.
This might say more about you than Uber or Lyft. Why is sitting in the back treating them like a "lower class citizen"?