Both disorders are technically mental issues, but with many transgenders, the disorder aspect overthrows their entire life. There's no drug for it, and the best they can do is cope. Since there is no drug that can solve the problem then sometimes the only way to improve quality of life is surgery.
However, there are plenty of transgenders who are only transgender because they are going through some kind of life crisis (like Chris) and want to escape themselves. Often therapy can resolve their issues without surgery. However, even if they still want to go through with it, transitioning can take years, what with all the hormones and even minor surgeries that are involved. That's plenty of time for the patent to change their mind on the issue.
It's similar with the transablists, but again, it's a quality of life issue. Still, once that arm is off, it's gone. There's not lots of pills and minor surgeries leading up to it. It takes less than 20 minutes to chop it.
And what happens when they turn out to be one of the kinds that are just going through some kind of life crisis and don't actually want to be transablists? You can't get an arm back, but you can switch back to being make and female in appearance.
As for those who do it for fetish reasons, screw those guys... or wait, don't. Don't screw them.