Why do they think the way they do?

For reasons best known to themselves, they simply do the search engines' wishes. There are some search engine optimization methods that the engines don't want to be used; e.g. hidden text, cloaking, doorway pages, etc. Even though the engines care nothing for any website or for any search engine optimizer, these people do the engines' bidding - and they preach it to anyone who will listen. Some of them probably adopted the attitude from others who frequent the smaller forums and, without thinking it through for themselves, they presumably accept that it's the only right way of doing things.

Most of these people won't discuss or debate any of the so-called unethical methods; they simply state that they are unethical, and that's that. When the occassional discussion does get started, they soon find that they have no answers to the opposing points, and they usually either disappear (stop posting) or resort to insults. Their reluctance to discuss the issues is understandable, because they don't have any rational or common sense arguments in their favour, except that it's against the engines' wishes. It's ok for them to follow the engines' wishes but it doesn't explain why they brand some things as 'unethical'. Unethical = wrong in a moral sense.

It is my considered opinion that the self-styled ethical SEOs insist that their views are the only right ones because they are simply incapable of doing real search engine optimization. They are capable of making websites and pages search engine friendly - something that can be learned in 30 minutes - but they have no abilities beyond that . Therefore, it would be in their best interests if all optimizers came down to their level so that they have a chance of competing.