Can a salaried employee be forced to work weekends?
Yes, assuming you consider the threat of termination of employment to be force. That's perfectly legal in every jurisdiction I'm familiar with - but I've lived my entire life in the United States, where labor laws tend to be more employer-friendly than in most industrialized countries. It might not be true in some places.
Assuming you mean literally forced, as in a gun to the head: Still yes. That's illegal, of course, but the question was "Can a salaried employee be forced to work weekends?" and not "Is it legal to force a salaried employee to work weekends?"