Sorry, whoever wrote this got it all wrong. No private car is obliged to stop in Cuba, it's the cars belonging to the state companies, with blue license plates, that must stop at the 'amarillos'. And the reason is they are supposed to be on duty, since they belong to the state, so supposedly you are not interfering with any private trip or whatever. Communism is illogical and all but not that much.
Oh, I see, sorry, you took it from an article. Well, that article got it all wrong.
- If you think it's wrong, just fix it. We are happy for any contributions, just go for it ;) Don't worry, it's impossible to break something here :) --MrTweek