Phillip Green doesn't give enough of a crap to sue. This isn't his market. Mumsnet panic as their market is Instagram to a large degree. Mumsnet bad press on Instagram = advertisers pulling out.
Phillip Green doesn't have that issue.
It doesn't make it right. Personally I'd rather no-one got horribly bashed, be it him or them. But constructive criticism and opinion should be fairly allowed across the board.
But yeah, it all boils down to the business model. Mumsnet know how fickle web users are these days and how much advertising revenue they need to stay relevant.