Maybe they think if they can prove trustworthy with this event, it will open the door to even more opportunities later.
By opportunities, I don't necessarily mean escape.
But suppose they are rewarded for good behavior to make it to the part of the prison where they can be released to go to a job during the day (work release).
If they spill all their candy in the lobby on this first outing, it could throw a wrench in the bigger plan and big picture.
It seems it would be much easier to escape from a job after working there a few weeks with no problems after the prison relaxes their guard even slightly as opposed to a chess event where everyone's on high alert.
To relate it to chess, think of it like attacking a square for the sole purpose of probing to create a weakness in the squares around the king.
Your opponent advances a pawn to parry the perceived threat while you patiently wait to lower the boom when you have a bigger advantage due to the weakness you worked to create.
Or say you're in a max security fed detention center and are being transported to a court appearance. The marshal puts the black box around your ankle and the cuffs around your wrists.
You've created some rapport with smiles and some friendly banter about the weather or hope they're not working him too hard, etc. you get the idea.
When he puts the cuffs on, your whole demeanor suddenly changes to a shocked look of hurt like you've just been betrayed.
You ask him,
"Man, these are a little too tight and the metal is cutting into my wrists, cutting off circulation. I think my wrists have always just been thicker than average. Got it from my father's side. Think you can relax their bite just a grip on here?"
(You have a 50/50 chance at this point depending on how well you "sell" the pain and how seamlessly it flows from the first rapport building.)
If it all flows really smooth and naturally, then it's been known to happen. I mean, one size fits all cuffs aren't going to fit the same on everyone and it's possible they truly could have been sized too tight around your wrists by accident.
I mean, they don't measure your wrists before applying them. The chance for human error isn't unheard of with that procedure.
Then you thank him, really appreciate it, etc. and change the subject or just leave it at that.
Now is the time to adjust your position in line.
You need to position yourself to where you will be seated in the back of the van and on an outside seat when the prisoners are loaded, but keep everything really happy-go-lucky, because this care-free attitude now will help you create your own luck later.
This accomplishes a few things.
The prisoners' heads seated in front of you can provide some cover once you're on the road. All you need is a little mobility in the blind spot and you're good.
Because you're on the outside seat, you're able to kick the back door open when the van stops to refuel or even at a stop sign/stop light.
There are many variations to this but there are no do-overs.
To get your hands out of the cuffs after they've been relaxed, you take your one hand with the other and with your thumb of the one hand around the base of the bottom thumb joint of the other and with your four remaining fingers around the side of your hand below your pinky,
You squeeze the base of the thumb in towards your inner hand and at the same time squeeze the side of your hand below your pinky in towards the inner part of your hand.
Now you tuck your thumb into the middle of it all and squeeze your fingers on top of each other until your entire hand is the size of just bigger than a couple of fingers.
You'll basically have three knuckles you'll need to slide out of the cuff.
It may cut your skin a bit, but if your freedom is worth it, it's worth it.
Once one hand is free, it's time to do the other.
You should space out the timing a few minutes apart and keep your facial expression the same throughout for the rearview mirror.
Also, you don't want to unnecessarily alert any of the other prisoners to what you're doing. You don't need any of them to become heroes for this adventure. Also it may make some of them nervous if they think they'll catch an extra charge for helping you or not telling the guards, so it's best to keep this one close to the vest.
The hardest part is removing the black box.
If you have to disarm the guard if you get caught after kicking out the back door, you may or may not have the opportunity to get the gun.
If you do and think you can do it, then you may be able to get the key to the box or otherwise do something to help your cause such as order them to drive the other way or something.
That's when it gets messy so you probably want to get out and somewhere out of sight before they make sense of what happened.
This only gives you a few seconds head start.
I don't advocate doing this and it's only speculation, in order to answer your question, but I will admit to have never been called Houdini because of my chess skills and leave it at that.
Good luck.
I know of a small town that organises every year a big chess tournament for all their citizens and the inmates of the local female prison. It is rather upbeat event and tourists come from far and away to get a glimpse of the satisfaction of the players. The big question that everybody has is why these women prisoners do not take the opportunity to escape, but play their games, enjoy the happening and all return happily to prison afterwards. Who can explain this?