I just went out and checked mine....
Anything forward and they stayed in the position I put them in. They came off reverse a bit. Mind you this is not running, its 5:20 here.
There are other safety devices on the zero turn. Get off the seat without the parking brake set, kills the engine. Bring the motion control levers in from their neutral resting position with the parking brake set, kills the engine. Get off the seat with the PTO running, kills the engine.
Keep in mind these are all JMO...
In relation to a car, taking your foot off the gas doesn't bring the car to a stop, it requires additional input from you.
In relation to the tractor, the HST returns to neutral so it wont run anyone over while the operator station is unoccupied and there is a switch to kill the engine if the HST isn't in neutral and the operator station is unoccupied. You cant get off the seat of the ZT, while the engine is running, without the parking brake set and you cant set the parking brake unless the motion control levers are in their neutral resting position.
These CUT Tractors also have Cruise... Which requires input from the operator to release. Think of the levers on the ZT as cruise control....
I think.... and Ive done the exact same thing... you are approaching this as an abnormality because its different than what you're used to. I'm actually kind of a safety XXXhole and I cant think of any need for the levers to automatically return to center... My first inclination in an emergency wouldn't be to let go of them, it would be to reverse course. Letting go of them may actually make the issue worse if you needed to get out of a situation... Say you run over a nest of ground bees... Let go of the levers and come to a stop or reverse course/speed up and get out of the area? Say you're on a hill and you feel like the ZT is going to roll, let got of the levers and come to a stop or make course corrections to try to move your center of gravity?
Again, not attacking. There is no malice in my tone... I just don't see where letting go of the levers to come to a stop would be a necessity. Now if the levers creeped forward or reverse with no operator input, then I could see a problem.....
But ultimately its got to come down to what you and your wife are comfortable with. Have you checked any other manufacturers? The Gravely units seem to be well regarded. I didn't actually put my hands on one but they look nice from what Ive read. The SCAG Tiger Cat II is right around the same price as the 700 series Kubota.