It's like this... (Score 1, Informative)

What is being sold is not the software. What is being sold is a licence to use the software. So if you want to resell your licence to someone else, go ahead. But that means you no longer have the licence to use said software. That's all.

I sell software in the form of licences. If someone resells their licence, I have no obligation to support the seller anymore. I do have an obligation to support the buyer, since they have bought the licence (less any time or licence renewal restrictions).

What's tricky is that, people being human, it's too easy to resell your licence and "forget" to delete your copy. The "threat" of no longer being supported doesn't worry people enough, hence publishers get more draconian about resale rights.