I think cutting the tips of actual feral cats is a good idea because it is quick and easy to identify those cats who have already been fixed. The ear notch is noticeable enough that you can see it from a distance and lets you know that the cat does not need to be trapped again. While ear tattoos would be a good idea the problem is that they wouldn't be noticeable unless the cat has already been trapped or you can get close enough to look in it's ear (however most feral cats would never let you get that close).

I don't know how your county runs the feral vouchers. I know some shelters you call in and order the vouchers, pay for them then you just bring in the cats once they are trapped. I think here it's something like $20 for a male neuter voucher and $35 for a female spay voucher.

Maybe they keep this policy so that people who are doing this simply because they want a discount will be discouraged from ordering the discount coupons to save a few bucks when the coupons should really be used to help spay and neuter feral colonies so that they don't continue to grow.

As for your situation it's hard to say, as for the vet I can understand him wanting to follow policy. These coupons state that they must have the ear tip done. But I can also understand why you are angry.