They can remove essentially the whole rectal cuff, as well as the mucosa that can develop cuffitis. There are two reasons this isn't usually done automatically:
1) Without a rectal cuff the anastamosis must be hand-sewn instead of stapled. The hand-sewn procedure is much more technically difficult. It's quite possible that most J-pouch surgeons have never performed one, and it's certain that most of them aren't very experienced at it. It's easy to screw up.
2) The outcomes are worse, on average. The risk of incontinence is greater with a hand-sewn procedure, and most folks with UC don't develop cuffitis. That's not very comforting when cuffitis is ruining your life, but every roll of the dice includes some possible stinkers. Even when rolling the dice was a good bet, it's not always a winning bet.
Pouch advancement surgery with mucosectomy generally solves the problem when the surgeon is carefully selected. That usually means traveling to a different city. It's another roll of the dice, of course, but it's usually a good bet. And for some folks the idea of any more gambling than absolutely necessary has become intolerable at that point. Whatever you choose, I hope it works well for you and you feel better!