There's this persistant belief that IBD patients take lower doses of Imuran/ 6MP than transplant patients, but it doesn't to hold true in practice.
Transplant patients have a loading period where they do take higher doses than IBD patients, but standard practice has transplant patients lower their doses on a maintenance routine after transplantation. These doses aren't much different than those for IBD patients. Meanwhile, many IBD patients have higher and higher doses to attempt to control their disease.
More or less across the board, the standard dose is 1-3 mg/ kg. I would not automatically assume that IBD patients on Imuran/ 6MP who were later diagnosed with cancer were taking higher doses. Too, one should consider that, as recent research has indicated, most IBD patients discontinue Imuran/ 6MP after a period of time, so long-term compliance is not that great, presumably lowering cancer risk. Although lack of compliance/ discontinuation of Imuran/ 6MP is a major cause of organ rejection, transplant patients generally do not have the "luxury" of discontinuing their meds thus prolonging their exposure to the drugs.
For UC, Remicade was initially considered a bridge to surgery rather than a maintenance med (and it's still used that way just about everywhere except the US). Couple that with its relative novelty, and it's not a surprise that there aren't many (any?) studies on long-term use. Until then, it's wise for the FDA to continue to release updates so that patients can make an informed decision.