Involuntary "Citizenship"

My parents escaped from communist Poland when I was a wee baby. And there was a level of actual "escaping" involved, the country was trying to prevent their leaving. Preventing one's citizens from leaving a country, despite their wishes, is a pretty infamous characteristic of totalitarian regimes, especially communist ones. Our media still criticizes North Korea for it.

How are the Western countries trying to stop people from leaving their state any different?

For that matter, can anyone figure out why the UK (and other countries) are trying to prevent their citizens from running off to the middle east to join IS? Shouldn't that be encouraged? It's gets dangerous radicals out of your country, and it makes them happier. AND it makes all their former neighbors safer and happier as well. What is the downside? Everyone wins. And you aren't faced with the bad publicity of attacking your own citizens within your own borders, which has always been a red flag of totalitarianism. Do we really want to become more like the very societies we've been taught are evil for so long?

What am I missing here?