Inspired by a comment on the crumpet thread.
I understand why people might take half a biscuit/doughnut/cake/pie, whatever. They are teeny tiny and can only manage a few bites. We all get that.
BUT why do they think it's acceptable to touch said item and put it back like someone else will want it?
If you do this, why do you do it?
Why don't you take the other half for yourself another time?
Why do you think someone else will want to eat half a squashed iced doughnut with your fingerprints in it?
Even more disgusting are the people that don't even pretend at hygiene and don't even get a knife, they just tear it in half with their hands and put the other half back 
Yabu - it's fine for people to touch up food items with their dirty fingers and leave them for others.
Yanbu - these people are disgusting.
And I'm talking more in office spaces etc than your home, but still, homes count if you have people other than your immediate family there.