Because they're all in it together, and because Sarah Harding needs zero encouragement to be anymore lairy onstage.
In all seriousness though, it's very considerate for Girls Aloud to put a ban on alcohol backstage on their upcoming tour, seeing as Harding has gone through the wringer enough times at this stage. She doesn't need any more excuses to keep filling her face with poison, let alone the rest of her body. And seeing as it's probably the last hurrah for the group, they need to go out on as high a note as possible.
Says a source to The Sun,'The girls have decided to ban alcohol altogether in support of Sarah. She's still dealing with her alcohol problem two years on, and they've decided to ban it out of respect to her. It's only fair - they don't want her falling off the wagon and going back to how she was after she split from Tom (Crane, her ex-fiancé)'.
And if they ever wobble on that wagon, they can always rewatch this clip of Cheryl puking in a bin for encouragement.