Forming the basis of means it is essentially the sole basis.
Forming the basis for could mean it is one of the factors forming the basis.
Agriculture is the main occupation, with tobacco and cotton crops forming the basis of much local industry.
The company continued to prosper, forming the basis for much of the commercial activity of colonial Canada.
Regardless, it seems there's a tendency these days to prefer for over of, especially in AmE.
A complaint shall be filed within one year after the date on which the (...) receives a grievance forming the basis of the complaint. No complaint shall be filed more than ten years after the date on which the misconduct forming the basis for the proceeding occurred. [emphasis mine]