This way the driver can put mail in suburban mailboxes without leaving the seat (while driving on the right side of the road).

Current US mail trucks are right hand drive so the driver can deliver to curbside mailboxes without getting out.

You are right that US drivers usually have the steering column on the left side

However, postal drivers get out of their vehicle all the time, and often on roads not meant to have vehicles pull over. Therefore them getting out from the right hand side prevents them from having to exit into traffic.