Maybe it is more about business model than technical details.
It seemed to me that license key is giving users access to a product, while entitlement allows users to use some sort of service (like 2GB storage a month)?
Both are part of the same hierarchy of Customer Software Management.
At the point of sale for a software product the customer has purchased a license key and explicitly (with the specific EULA details he agrees to) gets the immediate benefits and responsibilities of that contract/license key.
But the customer also implicitly 'inherits' any add on privileges and benefits (entitlements) of that software sale too... such as future free versions within a time period, volume licensing, discount pricing.. which he may or may not make use of.
Further research on this... Safenet company... http://www.safenet-inc.com/software-monetization/sentinel-entitlement-management/