I did ATPL groundschool about ten years ago, and the method of fuel management I was taught was based on airline practice, i.e. calculate taxi fuel, trip fuel to destination, fuel from destination to alternate, half an hour's fuel for remaining in the hold at the alternate, and then landing and taxi-in fuel; (not to mention any allowance for 'unusable fuel' and maximum permissable fuel load to remain under max landing weight). Obviously, you can't use this method on a Pa28, but it does at least provide the mental backgound for my fuel planning.
I don't have an issue with somebody doing fuel planning one way or another way, so long as it is realistic for the situation. (The "fuel circles" method is not good for "slow GA" because the wind is likely to be very significant, so the circles are not circles anyway).
What I think is the biggest issue is that in GA most people don't know their fuel flow accurately.
I know nuffing about big jets but I am pretty sure they have accurate fuel metering (totalising) systems. They probably also have accurate fuel gauges.
But they are also hugely capable weather-wise (anti ice, FL350+ ceiling, autoland, etc) so a "30 minute" (or whatever) reserve for something is actually meaningful whereas in GA it is all but meaningless because 30 mins' flight is unlikely to get you out of a bad wx area.
Piston GA needs bigger reserves.