I think if this bothers you, instead of posting on here, talk to your nanny and just have a conversation about how and why she did this. Probably perfectly reasonable explanation.
Your child will come to no harm because of it.
But trying to enforce such a strict policy may be harder to adhere to as you child grows up. And you don't want to give them a food issue, potentially.
I was once at a party where a Mum would not let her 4 year old daughter eat a lovley little pink fairy cake 'because of the sugar' not fair to see the poor child in tears and the mother looking all hard faced and smug. It's one little cake FFS.
There is a happy balance to be acheived eg: both DS's are having friends for a sleep over tonight, on the menu are chips and pizza and popcorn. But they have eaten a good balanced diet in the week, fruit and vege, yoghurts, fish, pasta, brown in fact which they prefer. I do insist on fruit for pudding for both and then maybe a bisuit or treat. Give and take.
So it's not the be all and end all is it, if your nanny did this? I agree a tom sauce is easy, using tinned toms, herbs, garlic etc, very quick.
So lighten up. There are many more challenges ahead of you as a parent and this sort of attitude is heading for stress and disaster for you both.
Good Luck.