Coeliac here and I generally only allow subs on things where it's extremely unlikely it would be subbed with anything gluteny — things like eggs or carrots. If I'm using a supermarket that allows comments with each item, I'll allow subs on GF bread and comment "gluten-free only please", because gluten-free bread will say so in big letters, and probably be in a different section anyway. I specify it because it's entirely possible that there are people out there who would prefer a gluteny loaf if there are no GF loaves, whereas I'd obviously rather have literally any GF bread product.
Anything else, I'm not going to expect the picker to have the time or the experience to carefully read labels for me.
Having said that, I'd have possibly made the same mistake you did with the fish fingers in assuming that the free-from sub was going to be suitable, but that's only because as coeliacs we're used to having almost the whole free-from range cater to us.
I've made plenty of mistakes with online shopping too — I accidentally left subs turned on for some crisps, and rather than subbing a different flavour of own brand or subbing Seabrooks or Kettle Chips, which I would've done if I'd been in store myself, they gave me Walkers, which I'm betting 95% of customers would've been very happy with. But I wouldn't usually even ask for subs on crisps even if I could specify GF per item, because it would involve the picker reading the backs of packets, which I don't think they have time for.
DP and I are both CEV so have had to spend long periods of time living off supermarket deliveries, and it's a real pain when you have dietary requirements. Frequently stuff turns up which gave no indication on the web listing that it had The Forbidden Ingredients, but when it gets here and I scour the label it's been reformulated to contain wheat (happened with a meat paste) or has a previously unmentioned May Contain warning, and nobody's got time at the door to read every label.