Yes, but then there is a prescribed compensation that everyone involuntarily denied boarding (IDB) is entitled. The vouchers is an attempt to avoid this IDB by getting someone to voluntarily fly later.

Plus, I am not a lawyer, but I think IDB means just that -- boarding. Once you board the airline cannot change its mind, not for commercial reasons (although it might be able to weasel out of that by getting everyone off and re-boarding without some people). Again, take with a grain of salt; this is just what I heard.