I agree, and I rather suspect that everybody on the forum does as well. Unfortunately it's not us that you have to convince!!! I suspect that, at some point, Tesla will have to allow this. What they're doing isn't really quite kosher and at some point some regulatory agency will have something to say about it, I'm certain.
If I had to guess, I'd bet that the issue is Tesla's rather extreme market segmentation and pricing strategy. If you can buy a 2 year old used 90kwh battery for 10K and put it into a new 60D, why pay the huge price difference between a 60D and 90D? They're exactly the same car after all. This issue will go away as the product line matures. Tesla needs to further differentiate the high and low end - with clear visual cues and features that only exist on the higher end vehicles. In the meantime they're doing what they can to protect their margins, dicey though it may be.