“We cannot accept responsibility for any self drive hire operators in terms of how they proceed to drive our vehicles.”
Tells me all I need to know.
They might have phrased it a bit more diplomatically (or just not pointed it out at all), but they're right in any meaningful way. If I rent a car and drive like a loon, Hertz or Avis can't be expected to bear any responsibility for it.
Sure, they can decline my booking if I've got too many points or convictions, and they can refuse repeat business if they find out what I've been up to, but they can't do an awful lot else.