In most European countries you only get reserved seats at big multiplex cinemas, stuff like Cinestar, NOS and so on.
On the European Cinema network [0], reserved seats is a long gone concept.
So not always a given that seats can be reserved online for cinema, depends on ones location.
I've been to several European Network cinemas and always gotten reserved seats
That's not true, I frequently reserve seats at the "Yorck" cinema chain in Berlin which is part of Europa Cinemas and has seat reservation.
I don't recall having been to any cinema in denmark ever that did not do assigned seats. They won't check if nobody complains, but is is printed on the ticket.
Here in Oslo, Norway I only know of the local cinematheque which doesn't do reserved seats. All commercial theaters have reserved seats, even for the small screens with just a dozen or so seats. Been so as long as I can remember, so several decades.
So yea, location dependent.