I agree with this, but also think that working for an employer that is "making a negative impact on the world" might _sometimes_ be the right choice. Specifically, it could be justified for two reasons: First: you might have more impact on the organization from the inside than from the outside (this is most relevant to people joining at a high/senior level). For example, Google seems to be making privacy far, far wor…
About double think, what is the difference between cognitive dissonance and your mind actually changing about a certain company or role because you now are more informed about it and choose to see it in a better light? People swear to never do things all the time until they do them. Does that mean they're evil people? Or does that mean their views have changed? And if their views have changed, who are we to tell them…
I think it's important to distinguish between two types of changes: changes in factual information versus changes in values. If someone changes their mind because they have more information, then that's basically fine (it could be bad if the new information is wrong or misleading, of course, but there's no fundamental objection). And working at a company does sometimes lead to this sort of change (learning about confidential info, for example).
If someone changes their views based on something other than facts, though, it means that their values have changed: something that used to be important to them no longer is, or something else has grown in importance. In my experience, this sort of change is much more common when someone works for an employer for a long period of time – it's not that they have learned important new facts about that employer, it's just that they are now culturally "on the inside" and don't want to think ill of their friends and colleagues (there's that cognitive dissonance again).
You asked "who are we to tell them what's wrong and right?". I believe that we _can_ legitimately criticize someone else's value system, but that's getting into (admittedly contended) philosophy.
Instead of wading into the philosophical debate, I'll just say this: Most people don't want their values to change (that's almost what it means for something to be a "value": you value it!). And (imo) most people significantly underestimate the amount of values drift that can take place for purely social reasons. Given that I care about living up to my values, I don't want to hack my brain in ways that make me stop caring about those values. And that's just as true when the change to my brain comes from working ~5 years with basically decent folks as if it came from brain surgery. (And, imo, smart nerds are especially at risk of underestimating how much their values can drift based on their peer group.)