The release notes for patch stated:
Most important, when running the USA DLC game mode, there was a bug which caused towns not to be completely American but mixed (all buildings, European and American, were enabled as a consequence of the bug).
This appeared to fixed when I started a new game today. Only USA streets were generated on map and also in ‘streets’ menu showed only US streets.
Sadly the problem has returned when I re-started my PC and the game yesterday afternoon. The map originally generated correctly as an ‘American only’ game, but on re-starting it now has both types of streets when cities grow. The ‘streets’ menu again showing both European and USA streets.
Sorry, looks like this need to be re-visited!