Weight and balance calculations for a 172 is something I actually know how to do! You basically add up all the weight in the plane (people, cargo, fuel, plane, etc) and determine a total weight. Then, you figure out where the center of gravity (CG) is based on how far each item is from an arbitrary location called the reference datum. Basically, if someone's in the back seat it affects the CG more than someone in the front seat.

The total maximum weight is a significant concern, and varies based on your CG location, but it's usually 2,300lbs. You can check this chart to see how it varies.

Assuming an average sized male as a pilot (160lbs), full fuel, and no bags at all, a single 250lbs passenger makes it pretty much impossible to take more than one other passenger. You can drop 41lbs (about 18%/7 gallons) of fuel to make one average sized male, one large person, and two small people (100lb each) work. Even then, they're carrying no bags at all.

According to this document the average American Samoan man has a weight of approximately 225lbs and the average American Samoan woman has a weight of approximately 215lbs. Lets say two average American Samoan woman and two average American Samoan men want to fly in a Cessna 172 together:

With full fuel, we're 211lbs overweight. With zero fuel, we're still 71lbs overweight. Drop one of the dudes and you can only about 25 gallons of fuel (63% of full).

With two 160lb people and two 140lb people, we can take 30 gallons of fuel (75% of full) along with almost 30lb of baggage and be in the clear.

Basically, the average weight in American Samoa combined with the low gross weight capacity of a Cessna 172 probably forces them to leave an empty seat on the airplane frequently. When you only have three fare paying seats, this represents a huge chunk of revenue.