If you open a front-load washer in the middle of its wash cycle, it would dump 5–10 gallons (editors note: however much that is) of dirty, soapy water all over the floor. That's bad - so the manufacturer designs a window so that you can see that the machine is empty of water before opening the door.
I guess it's for that reason. Dishwashers could have similar problems but they might have a lot less water in them at one time and with the type of doors they have you might not dump as much on the floor or something.