Teflon is less hazardous to the enviorment than bio bbs that aren't completely biodegradeable, as you said, it doesn't biodegrade, but it doesn't poison the enviorment either, I still recommend picking the bullets up on sight (after you get shot by them maybe), and maybe we can make a discount
(sadly thermoset plastics can't be re-used)
Still, the amount of sniper shots on a field can't even get close to the amount of BBs shot, the amount of pollution you add to your enviorment just by using plastic bags is a lot more than using teflon pellets.
Though, if these were bbs that are shot by aeg, it could become a problem by filling up the field.