Probably more of a PR matter than anything for the rental companies. It's been said that current owners are free to continue to use their vehicles as this gets sorted out.
It's a good point though...quite frankly, at the very worst, when VW announces its "fix", the rental companies should be mandated to apply the "fix" to the offending VW's in their respective fleets...better yet from a corporate responsibility standpoint, retire these vehicles...
Do they even rent diesels in the first place in the US? I can see Hertz or Avis having to deal with a lot of "I put gasoline in the tank" problems with diesels.