Has the field owner run in to pay all medical and then give you free play for life yet? Quick... sign here??
This really sounds like blatant negligence on the part of the field. So what do you think you're going to rename your new field?
I see the same thing in the scuba world all the time. Other shops that don't enforce safety rules, have bad instructors, leave stuff lying around the pool that people can slip and fall on, under staffed dive classes - then when someone gets hurt and a suit pops up, they act all surprised.
According to our insurance rules, you must walk the field checking for safety hazards every so often. The unfortunate thing here is it might be the field owner told his employees to pull the stakes, and they never did... but ultimately, the buck stops with the guy who owns a business.
Being in two high exposure / risk businesses, scuba and paintball, we keep a constant eye out for the safety side of things.
I do a briefing with my instructors, and the paintball employees will get about the same one. When I'm looking for employees, I have 2 major criteria... 1 is safety. Can they spot problems before they happen and take care of things so I don't have to constantly monitor everything myself. 2, Fun. Do they know how to make sure the customers are having a good time.
The mix of these 2 is sometimes difficult... Player removes mask on the field... you have to do the "Hey, please put the mask back on... gotta keep it on all the time your on the field... ok? Thanks man!" Not "Stupid!!!! PUT YOUR MASK ON OR YOU'RE OUTTA HERE!!!!"
But in your situation of someone leaving hazards on a field... not cool.