It could be in the original agreement that if you sell the car, you can only sell to someone who also agrees to being spied on.

I mean, they could try putting the onus on the seller.

They could try, but ultimately Hyundai is spying without permission. Hyundai could sue the original owner for damages that arise from not being able to spy, but I'm guessing they'd go ahead and keep spying when they shouldn't. The original owner can't be held responsible for that.