I think basically- people don't want their loved ones committing suicide. I think there's also a common notion that all people who do it are in some sort of temporary slump/ depression/ psychotic state. I don't think people even want to accept that a person could be thinking rationally and come up with suicide as a reasonable option.
So- they view us as vulnerable, susceptible to making impulsive decisions and impressionable. Couple that with a site which may not encourage suicide but it doesn't always discourage it- especially when a person seems sure about their decision. Plus, access to information on how to most reliably do it and they panic.
I think there is certainly a possibility that some of us would only CTB via a more peaceful method. Without knowledge about say SN or inert gas, I suppose there's a possibility that some people will be too afraid to jump, hang, slash wrists, lie on train tracks. Or, they may try a method that has very little chance of success. So, to their mind- they likely think providing this information makes it too 'easy'. I suspect they like it that the more brutal and possibly painful methods are enough to scare people away. They may even prefer to see someone maim themselves- so long as they are still alive. Kind of sadistic when you think about it... They put that much value on life- literally any kind of life. So long as they don't have to experience it presumably! You have to wonder if their opinions would change if the shoe were on the other foot.
Plus of course, there is the very real problem of minors. I'm perfectly convinced that you can be unhappy enough to want to commit suicide as a child. I was suicidal at age 10. I'm not so convinced we do have the emotional stability and ability to rationalise to be able to clearly think through problems though. So, I don't like the idea of minors being here either. Basically- I suspect their parents are terrified they will overeact to a situation, stumble upon an 'easy' way out and take it. Plus- to their minds anyway, be encouraged to take it.