In this case it's that Helix Native becomes NFR (not for resale i.e not sellable again) after 1st transfer, this directly affects the resale value.
Used software selling for 50% or less off retail is more the norm than the exception - just check other forums, like KVR Audio, that have 'for sale' listings.
There are a variety of reasons why and it varies from plugin to plugin - supply vs demand, restrictions on resold items (like becoming NFR), some companies don't offer tech support or upgrade opportunities on used items, etc..