They continue to trade for lots of reasons. First of all, the offer may not go through. I personally hope it doesn't -- I like my shares and would prefer to be able to keep them long term. Second, even if the offer goes through, it won't be until months from now. A lot can happen between now and then, and that includes individual investors' personal situations -- for example, maybe someone needs to sell in order to pay college tuition for her child. Maybe etc etc etc.
Until it is no longer HNZ, it remains HNZ. It's as simple as that.