Cancer is not a disease like the flu or E. bola, where there is a single quick and easy cause for the disease which you can locate and describe in a single sentence. Cancer consists of malignant cell growth, it is true, but why are the cells growing out of control?
The fact is that there is not one single cause. At some point along the way, some cell experience a mutation to some gene which resulted in a slightly higher rate of proliferation than normal. However, this in and of itself is not even enough to produce a tumour. It simply gives the cells a higher number of daughter cells, and consequently, a higher risk of having more mutations.
At some point, one of those daughter cells acquires a mutation which results in higher levels yet of proliferation, or a mutation which encourages angiogenesis, or (in the case of epithelial cells, for instance) a mutation which allows the cell to escape typical anoikis pathways and perhaps perforate its basement membrane without dying.
Even the process of metastasis requires multiple mutations to happen, and yet if a tumour does not metastasize, it is typically regarded as benign. The point is, cancer is not caused by a single mutation to an oncogene or otherwise. It is caused by an accumulation of mutations which permit the cells to proliferate much faster than normal, to recruit the body's resources, and to live in tissues all around the body.
In the case of the particular gene that you mentioned, the exact role of the c-sis oncogene in cancer prevention is still unclear; in fact, according to the OMIM page,
The protein product of the c-sis gene has not been identified.
...though the gene is very similar to platelet derived growth factor genes, some of which have been identified as mitogenic factors. Theoretically, therefore, it is possible that a mutation to the c-sis results in excessive mitogenic stimulation, resulting in an increase in cell division and proliferation. But that last sentence is speculation, not hard science.
We don't know, but it's possible c-sis encourages faster cell division.