I agree that it should be obvious that the issue at hand is patents. But that is just part of the relevant information.
One question is, does this legally solve the problem? If it is on very shaky ground, maybe it isn't worth adding the code.
Anther question is, what downsides are there? IANAL, but perhaps including this patch - which has no purpose but one related to patents - signifies implied agreement that the VFAT patents are valid (or at least such an argument can be made by the other side). Which might make any infringement 'willful', with significantly higher penalties.
All of this information is unavailable to the kernel developers.