I moved to the US at the age of 17, to go to college (on full financial aid). I graduated and stuck on for a PhD in computer science. American private organizations (i.e. Williams College) have spent $200,000 on my education. The NSF (which indirectly through my advisor's grant funds my graduate education) spends $80,000+ per year on my education. ($160,000 so far, and counting)
Let's leave alone the stupid fact that my visa (F-1) is explicitly a non-immigrant visa (if I say that I have an intention to immigrate to the US someday; I can be deported or denied re-entry or visa renewal). So time spent in the US doesn't count for squat. If I hypothetically wanted to stay in the US, I would have to go to the "back of the line" and get an H1-B.
I'm from India, so unless I qualify for an EB-1 visa (i.e. I get a tenure track position at a top research university) it's 5+ years on the Green Card waiting list. During those 5+ years on the waiting list I am tied to an employer. This, obviously, decreases my negotiating situation. If I get fired, I have 2 weeks to find a new job or get deported. If I switch jobs, the green-card wait time can in some conditions be restarted. I am at the whims of a ridiculous bureaucracy at all times (the few times I've dealt with USCIS for employment authorization for internships have been a nightmare).
The H1-B is a poor option. Forget startups.
In contrast, due to my youth and education I am eligible for a work-permit not tied to any particular employer in Canada, and qualify for permanent residency in a year or two. Same with Singapore. The UK used to be the same before the Conservatives came to power and halted all immigration.
To the xenophobes, I'm not saying I have a right to enter your country. I'm just asking whether it's in your best interests to severely discourage me from entering. I probably will leave when I'm done with my PhD. No. I certainly will leave, because to state otherwise risks deportation under current immigration rules.