Rich, a bit late in responding because I have been away the last couple of weeks.
While I wasn't given the same instruction, I was told that the major cause of pain after the operation was wind in the colon where it had been pushed around (effectively bruised) during the operation. And it certainly felt that was what went on. It would make a lot of sense to minimise this by eating foods least likely to provoke wind, and maybe your hospital had found from experience that it was helpful to patients.
I haven't heard that it is something that has had a proper clinical trial to test though.