It's a savvy negotiating tactic to start with behaviors based on an assumption of good faith, but only if you aren't cutting your own throat in the process.
I'm not all that great at job hunting, so I have hesitated to jump in to this discussion. But perhaps that says something important. I mean, the fact that I'm pretty good at negotiating generally, yet not so great at the job hunting process. Perhaps that is important commentary in its own right on some of the problems in the job market with asymmetrical info and how that can impact job applicants.
Interestingly, most talented engineers i know in fact operate the same way. They try to understand the perspective of the other person first (in a meeting, in a negotiation, etc) instead of assuming what it is.
Your framing of this viewpoint as an attitude problem does not look great. It's not an attitude problem, it's a way of operating problem.
You want to hire people who don't just say "well this other team that isn't doing what i want, so they are clearly stupid, etc". You want to hire the person who says "hey, i wonder why team x feels differently when we both probably want success", and tries to understand their perspective and how they can work together towards some goal.
That is what the most talented engineers do.
When you do this to a naive kid, you might win out. When you do this to a seasoned professional, you burn any chance of him assuming good faith (which is almost never the case to begin with, given that negotiations are inherently adversarial), and he's going to enjoy playing hardball with you.
Listen, I know how this works. And I tell your employees and potential employees because I like when knowledge, dedication, and skill enrich people, and I hate when lesser-minded snake-charming enriches people.
It is absolutely reasonable to enter a negiotiation with the intent to base the resulting partnership on mutual trust. But "I will renege on this offer if you attempt to negotiate" doesn't scream trustworthy to me.
Hell, I don't think they're even necessarily assuming that the worst is common. They are assuming that it's common for employers to not pay them fairly if they don't take these tactics. They aren't assuming you will do that, but they are just worried you might might be.
We live in a capitalist society where the literal goal of an enterprise is to exploit the labour of workers and pay them less than what they create in terms of value.
There are young graduates who haven't worked that out yet and there are veterans who know the game well. At no point is trust involved in this equation. Employment is a mutually beneficial arrangement where the worker's self interest to maximize their wealth is always orthogonal to the employer's self interest of maximizing their wealth.
Just be a grownup and negotiate a fair cut. I fucking hate employers acting like we have some kind of trust, or worse, friendship. We're in a business arrangement. The only mutual trust is that I will do what I can to make as much money off you as possible, and you will do what you can to make as much money off me as possible. So long as that arrangement benefits us both, we have a working relationship. There's a reason why you will find a paragraph along the lines of 'don't be friends with your employees' in every book for founders.
People who say stuff like you're saying fall into one of two categories: the naive who haven't yet figured out how the world actually works, and the disingenuous preying on the naive who haven't figured out how the world works. I won't work with either.