You have my sympathy with this
@Hollysdad as Cassie too is partial to a bit of poo.
I find that doing the training that I do lessens the occurrence, the more time we do retrieving and practice heelwork and lately stop whistle, poo hunting is reduced. I appreciate this isn't your main area of interest, as you love walking, so may not be helpful! But in high risk areas I do lots of "magic hands", clicker heel with sausage and occasionally ping pong recall. I find that very often she comes to me looking to do this in certain places, and it helps minimize poo eating. Occasionally she will play tug, but has to be in the mood.
I keep meaning to work harder on "leave it" but haven't put any energy into it yet! When we have a slip up I tell myself that she's doing a marvelous job cleaning up for everyone else! I agree with
@SwampDonkey , they have learnt to self reward. I guess that their nose and mouth are so important to them as gundogs - that the nose says you have to find stuff, follow me, and when they get there the mouth says you have to pick stuff up, and before we know it the habit is formed.