There's no reason for male teams to reject the transgendered. They're going to be inferior, not superior.
Your comment underscores your lack of knowledge of the sport. (Why would you want inferior players on your team?) The men who founded the league chose to make the question a non-issue by ignoring gender as a criteria for participation/membership. Also women have the equalizing advantage of a lower center of gravity applying more force with their hips and arse and not accepting shoulder hits which men instinctively block with.
If women have a biological advantage in roller derby, then it's 'unfair' for an all-male team to play against a mixed-gender team, and it's the women's teams who should accept any male that can prove himself to be at least as good as their least good player.
I suspect, though, that, like almost all sports, men do have the biological advantage. Look at the speed skating records for men and women.
If it were not ubiquitously acknowledged that men were better, they wouldn't gender segregate sports in the first place.