This wasn't a criticism of the US. Look around the world at all the despotic regimes, or places with Dubai where you have a large population of migrant workers who have their passports seized.

The first basic tenet is throwing up barriers or blocking departure. The US doesn't really worry about people leaving - way more people want to enter then leave.

Seasteading has a very implicit hazard here though: there's an in-built ability to make leaving difficult (same with the mega-yachts and the like - when you happen to very rarely be in your employees home countries, you've created a rather effective barrier to them being able to depart).

I'd imagine restricting immigration would solve 99% of these problems. If someone wants to leave, then let them - it may just be hard to get back. If you have a place where the net desire is to leave, then you have problems that will not be solved by merely holding the passports of unwilling citizens.

I think similar issues would arise in space. I suspect living on a spaceship would be like being in prison

Very much so. Even with the best of intentions, the opportunity is going to be there and just way too easy to exploit.

Getting on a spaceship where it's an 8 month return journey from Mars, with alignments for transfer happening every 2 years, is going to be taking a hell of a risk with whoever control's the destination's intentions.

US and EU citizens could visit most places on a whim, but leaving and staying permanently elsewhere could be a lot more difficult -- even if you were an American who didn't want to go farther than Canada. (After Trump's election, there were more than a few Americans who found that a very unpleasant surprise, for some reason.)

And as we're seeing on the US southern border right now, the rules are very, very different for people who want to pick and leave, say, Guatemala.

There is a difference between "not being able to leave" and "not being able to enter", your examples are of second kind. Currently there are very few places that prevent people from leaving, e.g. North Korea, Turkmenistan, Xinjiang for Uighurs. East Germany and USSR in recent history.

Most countries have laws that strictly limit who can entry. What good is the theoretical freedom to leave, if there is no where to go?

Compare someone from Venezuela and North Korea. Millions of people managed to leave Venezuela and stay alive, unlike millions of people from North Korea who had died from hunger in 90s despite having neighbours ready to take care of them.

But if you think that there is nowhere to go, you need to support seasteading, because if it succeeds there will be many new countries that will be happy to take new citizens.

Does the perspective on the boundary matter if the end result is that you can't cross it?

And someone from Mexico? Venezuela?

Kinda sorta. Yes, you a free to run off to anywhere in the world. Your passport protects you even in hostile countries. However, you are expected to return home and pay taxes. To leave the United States is an expensive affair if you want to truly leave by giving up your citizenship. To work abroad is expensive too if the foreign country has a lower tax rate. As a US citizen you are free to roam, but never to change your home.

This. And I suspect the United States are amongst the easier places to leave, along with the European Union.

In both cases, the populace are free to roam and settle within the borders, but moving outside the borders is a whole different matter (with a few exceptions, but there are always exceptions).