My thoughts would be because if use are using it for a plantar's wart or something on the feet it could result in a small wound first and a small wound on the feet of a diabetic or someone with poor circulation could become a more troublesome wound. Talk to your doctor about it. They will know for sure. Maybe even a pharmacist could answer this question as well.
As Yin and Yang suggested:
(For diabetics), the greatest concern is with salicylic acid, which acts to intentionally create a wound..... Since hyperglycemia inhibits granulocyte activity and lymphocyte function.