I agree with the other commenter - to people reading, this does not apply in many places. Maybe in massive bureaucratic old-school tech firms, but not younger ones like Airbnb etc.
It is completely legit to see it differently and to have a completely different experience. All I can relate is what I have seen in the last 25 years in the Bay Area. Having seen the effect in action early in my career (both people opting out after they said they would and other sorts of moves which affected relationships) I sought to understand it and came to my current appreciation of the various points of view.
Before I posted I had checked in with a friend of mine who is a contract recruiter and they seemed to feel my understanding of the situation was both correct and that this effect is fairly universal. It doesn't mean that it is of course. The next time you have a chance to talk with someone, in a safe and informal matter, who is in the hiring 'food chain' of something you consider "good" startup, ask them what they think. Nothing feeds a discussion like good data.
The tech community has fairly high mobility; people move from job to job through out their career. Often times they will segregate into an even more exclusive sub-community be it 'networking person' or 'security person' or 'language person' etc.
Companies spend a lot of time and money recruiting, training, and evaluating their engineers. These engineers are the "secret" to the success or failure of a company's efforts to execute against their vision and plans so companies spend time and effort to understanding as much as they can about engineers. And when they can't understand enough they count on 'sourcers' or 'recruiters' to understand as much as possible about the engineers.
As I have worked at companies, both large and small, and built up companies from nothing to something. Every single one has encouraged employees to refer their friends knowing that good engineers know other good engineers, and they know bad engineers as well.
As a result of these forces, what emerges for any given industry, is a sort of ranking and rating system for pretty much every participant. There may be six degrees of freedom between any two people in the world, but between two engineers in the same specialty? It often seems it is often 2 or at most 3 degrees. As a result there is a giant sort of collective knowledge about people in the space after they have been in the space for a few years.
Now consider what a company is going through when hiring. The manager may have had to argue with their management to get a hiring requisition 'approved'. They probably interviewed many candidates and had group meetings with the interviewers over plusses and minuses, skills and deficits. They winnowed that set of candidates down to a preferred choice and made them an offer. Which the candidate accepted. It's a successful process and probably the manger tells the team the good news and everyone is looking forward to having some additional help with the project. If the recruiter/hiring person is on contract they are excited because they are looking at anywhere from 10 to 25% of this new hires annual salary as compensation for getting them hired. Perhaps one of the employees in the group made a referral and they are excited that they might be getting a referral bonus.
Now the new employee calls them up and says "Sorry I got a better deal elsewhere so I won't be joining you after all."
The expectations are dashed, the hiring process has to be reset to the initial square, perhaps a strong alternate choice has been missed because when they didn't get the offer they accepted an offer somewhere else. And the recruiter isn't getting their big payday after all.
This lays down a lot more 'hurt' than young engineers can really appreciate early on. And that 'hurt' can, and will, affect their future chances. Is it black balling? Yes? No? Maybe? It is a natural consequence of people being let down they lose trust. The manager, the recruiter, the other members of the team that interviewed this person, all of them now are a walking and talking 'negative signal.' And they will be for possibly several years to come. And since the community is generally more interconnected than you might imagine it is, the chance that at your next job change this event will come out and have an effect. A good team would bring it up and ask about it, but many teams (especially ones where there are many choices) might just move on to a different candidate.
If you are exceptionally unlucky and you decide to not work at this major tech company and then your friend who pitched the startup can't come through. You are doubly in trouble (can't go back and don't have a job) not to mention a partially transferred visa.
The smart thing to do is to take the job at the Major Tech corp, spend a couple of years saving some money and learning what you can, and then joining your friend with the startup if they still look like a viable offer. That strategy maximizes your future value. You have additional work experience, your friend knows that you will stick with your word even when you might not want to, and you will have a bit more knowledge of how well your friend can put together a startup or not.