According to Wikipedia, as of 2018 there were 5.8 million Puerto Ricans living stateside. The population of Puerto Rico is around 2.8 million. There are more than twice as many Puerto Ricans living on the mainland as there are in Puerto Rico.
The myths about the negative consequences of open border policy have no basis in reality.
Most people don’t migrate away from their home country/state. But even if they did, it would be of no negative consequence for the place they migrate to.
It should be clarified that this is because they are US citizens. Everyone in North Dakota can move to DC too.
Supposedly allowed and not at all welcome. Like most affluent countries, Sweden doesn't actually want poor people immigrating into their country. So they make things very difficult for those people. It's also why they shut down the big immigration flood, their society was highly intolerant to it.
Americans - commonly ignorant to the details of rest of the world - would be shocked to learn how regressive Scandinavia can be. From very strict immigration to virtually bulldozing minority neighborhoods in Denmark [1][2], to attacks on immigrants in Sweden and burning immigrant camps, to large political parties derived from former neo-nazi groups.
From Reuters in 2015 (the Romanians got the message):
"A series of attacks in Sweden on beggars, many Roma, has highlighted a dark side to a country considered a bastion of tolerance but where the far right has been gaining support by claiming society is under threat from waves of immigrants."
"An influx of thousands of mainly Roma migrants has shocked affluent Swedes, with beggars now a common sight outside supermarkets, IKEA stores and subways in the capital."
"There were around 300 reported attacks on Roma in 2014, up 23 percent on the year before, according to the Swedish National Council for Crime Prevention. Police say the figures underestimate the scale of the problem."
"The government reckons around 5,000 migrants, some of whom also come from Hungary, are in Sweden begging. Many live on the street or in squalid camps. In recent months, attackers have thrown acid at beggars and burned tents and caravans."
"Many migrants in Sweden say they want to work, but lack of education and language skills make it impossible for most, leaving begging as the only alternative."
https://www.reuters.com/article/us-europe-migrants-sweden/sw...
[1] https://www.nytimes.com/2018/07/01/world/europe/denmark-immi...
"When Rokhaia Naassan gives birth in the coming days, she and her baby boy will enter a new category in the eyes of Danish law. Because she lives in a low-income immigrant neighborhood described by the government as a “ghetto,” Rokhaia will be what the Danish newspapers call a “ghetto parent” and he will be a “ghetto child.”
Starting at the age of 1, “ghetto children” must be separated from their families for at least 25 hours a week, not including nap time, for mandatory instruction in “Danish values,” including the traditions of Christmas and Easter, and Danish language. Noncompliance could result in a stoppage of welfare payments. Other Danish citizens are free to choose whether to enroll children in preschool up to the age of six."
[2] https://www.npr.org/2020/08/15/900874510/facing-eviction-res...
"Facing Eviction, Residents Of Denmark's 'Ghettos' Are Suing The Government"
Sweden and Denmark are easy to point out as they have very racist and discriminatory policies on a national and local level. But Iceland and Norway also have terrible refugee policies and rampant xenophobia (especially in more rural setting).
In the US, over 3 million Puerto Ricans are now allowed to migrate to the mainland US (say to DC). The borders are completely open to them. In Europe, similarly, around 20M Romanians are allowed to Sweden. But they don’t.
Just because people are allowed to migrate, it doesn’t mean they will. And in the case of an overwhelming majority, they won’t.