This isn't quite the case anymore. Studies have shown an increase in stomach cancer after an appendectomy but this is attributed to the H. pylori infection. The H. pylori is also the aggravating factor for the inflamed appendix. Colon cancer in populations actually decreases after an appendectomy as the appendix is the initiating site for many colon cancers. (Appendectomy during childhood and adolescence and the subsequent risk of cancer in Sweden. Pediatrics, June, 2003, by Judith U. Cope, Johan Askling, Gloria Gridley, Adam Mohr, Anders Ekbom, Olof Nyren, Martha S. Linet and Boffetta P. Infection with Helicobacter pylori and parasites, social class and cancer. IARC Sci Publ. 1997;138:325-329)Although the presence of hematopoietic and lymphoproliferative malignancies confounds a direct causal relationship. There may in fact be a preventative relationship between appendectomies and colon disease although this is still under debate. (Does Appendectomy PreventUlcerative Colitis? Reviewed by Douglas K. Rex, MDIndiana University School of Medicine, Indianapolis, IN[Rev Gastroenterol Disord. 2001;1(3):160] and Appendectomy and Protection Against Ulcerative ColitisAndersson RE, Olaison G, Tysk C, et al. N Engl J Med. 2001;344:808–814.) Again, this may be related to the removal of a possible instigating site. There was some evidence for the relationship you mention 20 years ago (A case-control study of risk factors for large bowel carcinoma, Vobecky J, Caro J, Devroede G. Cancer. 1983 May 15;51(10):1958-63.)but it was found that these populations used there was an overall greater occurrence of colon cancers. It is now thought that the increase in cancers is due to an induction of tumor metastasis from the stress of the surgery. (Increased surgical stress promotes tumor metastasis. Tsuchiya Y, Sawada S, Yoshioka I, Ohashi Y, Matsuo M, Harimaya Y, Tsukada K, Saiki I. Surgery. 2003 May;133(5):547-55.)
Basically, the trauma from the appendectomy causes small pre-cancerous or early cancerous area to become very active in the abdomen.