Because she is so obliging!
Fresh from stalking the streets of New York as a vampire during the Atonement screening, Keira Knightley shows her obliging side by stripping down anytime – as long as you ask, that is.

Appearing topless on Interview magazine albeit with thin suspenders providing some bare cover (not that there’s much to cover anyway), Keira provided an enlightening response to the publication’s questions:
How was the photo shoot?
We ended by taking my clothes off again. How does that always happen?
Why do you think?
I don’t know. Because I say yes, I suppose.
So there you have it. Keira Knightley is an agreeable star and charitable from the heart. If you need her without her clothes on, just ask.
Keira, how about doing a nude shoot for SCN?
She can take off all her clothes and no one would take a second look – there’s nothing there!!!
is that why those suspenders are slipping? lol