"reasonable whatever" is often used in laws/courts/contracts. Lawyers and judges are used to interpreting this. If Facebook were to sue you, you could start talking about it as part of your defence.
Additionally, Facebook needs to be seen to be reasonable and have a proper 'whitehat' policy. If they start being mean and dictatorial here, then there will be a breakdown in social trust. People won't report bugs to Facebook, people will sell vulnerbilities on the black market. People will release exploits before telling Facebook. It will, eventually be bad for Facebook.
Additionally, Facebook needs to be seen to be reasonable and have a proper 'whitehat' policy. If they start being mean and dictatorial here, then there will be a breakdown in social trust. People won't report bugs to Facebook, people will sell vulnerbilities on the black market. People will release exploits before telling Facebook. It will, eventually be bad for Facebook.