I was told by my doctor that it would Not be possible to develop any fistualas in the rectum/anus area if these parts were removed. (I had a perianal fistula once many years ago, it came out to the skin just near my bum hole, and developed into an abscess--sorry if that is too much info...!).
Anyway, the idea is that since all the diseased digestive tract tissue has been removed (CD and UC only will affect the digestive tract tissues), then there is no place for inflammation to begin, which is the root cause of a fistula.
This, at least, is what I was told by my doc. (I was happy to hear that, as my fistula and resulting abscess was probably the most painful thing I have ever gone through in my life. When I went to have the abscess drained, the surgeon told me he had never seen one so big (lucky me, what do I win?!). The whole side of my bum was swollen up...!