Fuzzy on Fungible--Dilbert

Hey guys, I was just looking at the Dilbert cartoon and I'm a bit fuzzy on a different economic principle. It seems to me that if Dilbert spends less money on gas, then whoever sells him gas is getting less money from him. Sure, someone else can buy the gas he doesn't, but he's not giving as much of his money to 'terrorists', just like he says. And, if he has an alternative fuel (say biodiesel), then he can buy that and know he's giving his money to farmers, US oilseed processors, etc. That becomes very attractive when the alternative fuel costs the same as gas or diesel.