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Legally it does not really matter where I bought the product.

Yes it does. It's the 'CS 757 box sold by another company', so e. g. refunds and things like a guarantee can only be processed by another company.

While I personally don't know why the update channel was separated for download and (another company) box customers and, like you, raise an eyebrow on the image damage it causes, the legal side is defined in the contract between another company and CS alone. We don't know that contract, but it's no new 2011 thing to define different service and update channels in regard to e. g. software products or versions of them.
I don't applaud to such policies, but, legally, they aren't a problem.
The product's name does not act as a valid indicator of any legal background.

From your car example, I think you generalise some aspects there, which can't be summed up easily. Even a car manufacturer can refuse certain services when you've bought the car in another country (maybe with far higher taxes, so the car's price itself was much lower) and want to operate it with full legal coverage in the other one.
Product liability will be in place all over the planet, but certain update, guarantee and service aspects may indeed be limited, even if your car has the same name as the one from your neighbour.

So as much as I don't like to see the another company box owners being stuck with 4.4, the legal side of all that is very fixed and valid in my eyes. It would be nice to see another company and/or CS taking a step towards those people of course.