Is there some particular reason, why anki is or should not be able to import these kinds of decks?
Judging from the error text, the only thing “wrong” with these decks is the dublication of card templates used within the deck. Such dublication of card templates could be remedied after importing the deck, either by changing one of the card types to ask for something different than the other card type, or by deleting one of the card types.
By not beeing able to import these decks, one must wait for the original author to modify the deck, which is not ideal in my opinion.
You should be able to import it if you enable the new importer in the preferences screen, though the duplicate card template will still need removing, and ideally the deck author will fix the problem and re-share.
I agree with you, that it is ideal when the original deck author properly designs the deck to not include duplicate card types. But sometimes deck authors just vanish and cannot be reached. In this case, being able to import a deck with the new importer and making the necessary changes yourself is the next best option.