75 and 80 kgs looks like very conservative figures
Clearly not an USA airline :-)
If anything those are a bit on the low side given the fast food culture in this country.
The other factor would be the significant extra time it would take to weigh/explain why to all the passengers.
I have heard of small commuter airlines moving people around to improve the cg, still has to be a bit disconcerting to be asked to move to front due to one's size.