Vouchers are valued at 50 DEC each, which is roughly equal to the current market price. For some players that may be worth it and some not. I believe that many players have excess Vouchers that they would prefer to spend on a promotion like this than sell on the market, but if not, then that just means more DEC will be burned, which is fine.
Also, the purpose of DEC-B was that it can be used in place of DEC any time DEC tokens will be burned. Players took a large opportunity cost converting DEC to non-transferrable DEC-B when the token needed support during the bear market, and it's important that there are opportunities to take advantage of that.