I'd expect the discussion to be different if it were a part 91 Gulfstream.
The Gulfstream isn't really subject to operating under a system where profit margin matters. Not saying they'd throw away money, but if you've got one jet that flies probably a lot less per year than a 777 does, then it doesn't matter so much if you take a bunch of extra gas each time you go up.
If United takes several tons of unnecessary fuel on top of the reserve/alternate/etc. fuel for it's entire fleet of 777s all year long, it's going to add up and will result in a lot of wasted money.
The two scenarios are not similar.