[lostcolony]

Inform. Even in such a culture, one person refusing to do it (not rudely, just passively; go home at the right time, not checking stuff) is enough to change things. We know from every bit of research that 60 hours is not more productive than 40 hours. The one person who refuses is going to shine. And their good habit, if they know to commit to it, and to socialize it, will help empower the others.

I know as a team lead I've tried to empower the people under me to manage their own time, and actively encouraged them to ignore workplace communication after hours