Nothing really.

Here in Norway they will be captured, and the police will handle them[1].

If the owner does not claim the captured dog within a week, the police will find a new home or shelter[2], or put it down.

[1]: https://lovdata.no/dokument/NL/lov/2003-07-04-74/KAPITTEL_3#...

[2]: https://www.dyrebeskyttelsen.no/hjemlose-dyr/

Nothing stops people from doing that but people do not do that. I've never in my life seen a feral dog here in Sweden, even feral cats are quite rare. I doubt they would survive a winter where I live so that would drastically limit their population.