Are they all this way, or do they vary based upon (length/ride leader/group)?All ACA guided tours have fixed start and finish dates, but the intermediate stops are usually tentative (except where accommodations have been booked in advance). Many people book their flight home before the trip starts so they need to be sure to get there on time. See http://www.adventurecycling.org/guided-tours/compare-tours/
When touring solo or with friends, some people also plan a fixed finish date. Others like to keep things open until they get all the way to the end. Still others wait to plan their transportation home for when they get close to the end.
Generally on long tours, there is plenty of opportunity to make up time if needed, so it's not very hard to hit the finish date unless some emergency comes up in the final days.
Long long long ago when I rode around Europe in the summer of 1992, I had a return plane trip purchased in advance. I picked a return date ticket when I bought the departure ticket. There was 3.5 months between start and end date so plenty of flexibility on riding in between. Easy to arrange arriving in the departure city on the right date. I think with flying to the start and end, the ticket price is much cheaper. Round trip ticket is cheaper than buying two single way tickets.