Because their current practice can only be described as fraud.
Despite what the fine print says (which I'm sure explains they're allowed to do this), most people are under the impression that they are buying a product, much like you would in the store.
To someone who believes they have bought/purchased something, it's like discovering that one day Amazon or Disney snuck into your home to steal your DVDs so they can re-sell them to you again next christmas. But then you read the fine print in the contract and realise that clause 14.2 (a) stated that they're allowed to do this.
The fix is for them to state "you will have access to this video for a period of 12 months" or similar.
I try to buy DRM-free where possible, but most of my friends don't care or don't know any better.