"Extra luggage is more work for the airline". Here's one scenario I came up with: a student from Australia does a high school year abroad to the US, joins the school football team, which places first in state that year. With a growth spurt plus putting on muscle weight that student ends up 15 pounds heavier on the return trip 10 months later.

Does the student have to pay more money at the gate in order to return home? If that student does not have the money, does the student have to leave luggage behind in order to make up for the weight? Will that be bad publicity for the airline?

Or the other way: suppose someone has a round-trip ticket from Australia to Maine to fulfill a dream of hiking part of the Appalachian trail. During the hike, that person loses 20 pounds. Does the airline reimburse the person for lost weight? And if so, is it in dollars or euro? If not, is this rightly seen as a penny grabbing method by the airliners?

If someone buys three bottles of wine at duty free (about 12 pounds) before flying back to Australia, will the gate agent charge them for the extra weight?

All-in-all, it seems like a bad idea for the airlines, and for relatively little money per flight.