There's nothing particularly interesting or surprising about it. The barrier is there to protect the robots from damage, and to protect the spectators and the stadium from damage. These are experimental machines and weigh enough that they could cause damage or injure someone.
The stretchers are for the same reason, basically. The robots have to be autonomous, with the only remote control being a kill switch to make them inert, for safety reasons. To prevent the teams having to go onto the track to recover their robots, they simply stretcher them off. Saves time, and helps control human access to the track. Similar to motor racing where cars are recovered by track staff, not the teams.
These are all long established safety rules for any kind of event involving machines, or even equipment like swords and javelins used by human competitors.