No, employees can opt-out and make trades as they wish. We add extra fund options to each plan to make sure that this is possible.

That said, we've found that most 401k users don't want to bother with analyzing and choosing between dozens of confusingly-named funds. They're much happier (and better off financially) with a solution that automatically gives them a globally diversified, low-cost, regularly-rebalanced portfolio without doing any of the work.