1. The rules say so, but none of the wings are strictly rigid. The FIA use tests to make sure the wings are legal. Since all teams pass the tests, then you couldn't call a particular team a cheater.
2. Ferrari has no idea how flexible other teams' wings are, they focus on their own car design given that it fits the rule, and they succeed in doing so.
3. You couldn't blame Ferrari for gaining an advantage (if any) with their wings, since they don't know whether other teams' wings are more flexible than theirs or not. FIA should improve the rule by bringing in more specific tests regarding the flexibility of wings. If anyone's to blame, it's the FIA.