I think it's simply a case of not crossing the line and perhaps the difference is that mumsnet deletes things that are potentially libelous when it's reported to them whereas it seems that the owner of tattle does not
Giving your opinion about content someone has chosen to put on there's fine.
Not liking what you see is fine.
Saying you don't like it is fine.
Saying someone really pisses you off is fine.
People aren't always going to be nice and positive in their opinions, and there's really nothing to be done about it.
A site just can't allow content that accuses people of illegal or reputationally damaging things (iyswim) without evidence. And possibly even if there is evidence. I expect there's legal stuff around that too.
Tattle just needs to delete stuff that crosses that line when the subject of it makes a complaint. Maybe get legal advice on whether what's being said is something that needs to be deleted for legal reasons.