The frequently unspoken tenet of all modern governments and organizations, is that you are generally not free to leave (pandemic aside).
Every place as it's first defensive measures tends to the resort to the same idea: prevent people from leaving (so, take them prisoner). Because if people were truly free to leave, then good luck exploiting them.
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).
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.
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).
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.
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.