Am I going to be devestated if my kids get hurt operating the tractor or the backhoe, yes. (son 10 and daughter 16)
Am I going to second guess why they were doing it, and should I have let them? absolutely.
But am I going to stop them from doing it? Nope.
I will not stop them motorycle riding, nor unicycling, nor trying to horseback ride at a freinds house or (most) anything else. Will they go Kayaking with my dad, and parachuting with my stepmom's boss? Yep, will I be nervous as all get out, yep..... I will also send them to Germany again this summer, put them on a plane and kiss them goodby and know they will be driving tractors in the field, and riding on all sorts of tractors and equipment (matter of fact, all the tractors I know of there have jump seats) and will I be nervous, Yep......
Was that kid joyriding? looks like it too me, but is that really the point? Do we do everything only when "needed" I would venture to say far more kids end up hurt and hospitalized (certainly in my town) by bicycle vs. Car accidents, but I still let my kids ride their bike on the street. (and 5' unicycle for that matter)
I guess there is no point in my dissertation here, other then we all have to judge for ourselves if something is "too" risky to do.
Then again, I am the guy who felt the tug on my leg of the backhoe tire, while I was standing in the door showing my 10 year old how to operate it. Would have been a bad memory for him if he had drove over his dad

(and his dad would not have been so keen on the thought either to be honest)