Eating fast food is one thing that comes to mind. You never heard them refer to "going to Burger Chef" (I am sure they could not use real names on the show) or having any type of fast food.
There was an episode where Marcia got Jan a job who ended up outperforming her in a, get this, SODA SHOP! Who the hell had soda shops in the 70's. It didn't even look like a Baskin Robbins or Dairy Queen. It looked like it was straight out of the 1920's.
The show was shit, designed to sell shit.
You make an interesting point. Soda shops were a 50's, maybe even a 30's-40's thing. With the prominence of fast food they sure as hell didn't exist in the 70's.
Often there is a lack of realism on tv because scripts are written by an older upper middle class generation out of touch with current teenage reality.