It isn't. There is NO SAVING SEATS. I know it is a polite habit but get over it.
The flight attendants repeated this over and over AND kept saying the flight is full take the first available seat.
I pay $12.50/per person so all my family can sit together towards the front in rows 5-10. However, the woman across the aisle kept saying to everyone "My husband is sitting there." No he wasn't. They only wanted to pay for one person on Early Bird while he had a C boarding pass. She brought the bags on and stuffed the overhead and then saved his seat.
The flight was full and I mean no empty seats at all, every middle seat was taken.
Easily 1-2 DOZEN people said "Is this seat taken?" (We were in row 7, so she got asked that much) Her reply "Yes, my husband is sitting there." After the first dozen my "It is all open seating." earned me an elbow from my husband and a let it go.
My advice. Throw your small bag on an empty seat, put your overhead if you need one (with SW we only travel with under the seat bags. Bags are free, so why worry about overhead space. You'll have more room towards the front since the overheads there fill up quickly. Any protests should be met with, "Talk to the flight attendant."