Fly between Berlin and Paris, and your luggage and ID will be checked at the airport multiple times. Not because you're not free to move, but to check you and y

Fly between Berlin and Paris, and your luggage and ID will be checked at the airport multiple times. Not because you're not free to move, but to check you and your luggage for security concerns, and sometimes even precisely to check if you're free to move (e.g. flying from the US to Germany as a European, how does one know you're not a Nigerian who has a tourist visa to enter the US, but no visa to enter Germany, rather than a German returning home from the US? By checking your ID) Having to show ID or getting inspected doesn't violate the notion of free movement in the way that this term is used. It's just that it's not commonly done in most modes of transport between most borders, e.g. cars between the Netherlands and Germany. But the UK is not alone in this, take the eurolines like I have half a dozen times (a bus network commonly used by poorer people, including poor immigrants) from e.g. the Netherlands to France and you're guaranteed to run into checkpoints. And if you've got the right ID for which there is freedom of movement, you're free to move.