They have to act conservatively, @mummytwoshoes - because if they accepted a child, whose parent said 'It's not a tummy bug, it's teething', and the parent was wrong, D&V could spread through the nursery like wildfire.
Look at it from the other side. Imagine that you have sent your perfectly healthy child to nursery, and another parent has sent their child in with D&V, and your child gets it. Your child ends up having days off nursery, and you may have to take days off work, if you can't find other child care that will take a sick child. Would you be happy that that parent had put you and your child in that position? I doubt it.
And this isn't a hypothetical situation - it happens. I have seen plenty of accounts in here of children being sent into nursery and school with infectious illnesses, by parents who don't care if their child infects other children or staff.
The nursery cannot take that risk - so they have to apply the guidelines equally to everyone.
And I do know how frustrating it is. Ds1 has been prone to migraine-type headaches since he was very little, and when he gets one, he throws up. He then has a sleep, and wakes up feeling better. I knew 100% that a headache-related vomit was not infectious, but I still had to keep him off school, if he'd been sick.