I don't think that anyone has ever 'decided' on that... we just don't have them and I don't believe we ever had. For a native speaker of Polish, it's having articles that is a strange idea.
I think a word like Duolingo should have all forms identical, but I cannot state it as a 'fact'.
That simplifies things – edited!
Is that perhaps a rule for company/trade names and perhaps some other proper nouns?
No, rather the endings. And -o ending is problematic: in theory it should undergo declension, but in so many examples it just sounds weird to Polish people...
For example, let's take Cristiano Ronaldo - until recently I had no idea that this name should undergo declension, I have never heard it or read it in any other form, but apparently it should be. For example "Cristiano Ronaldo's goal" should be "gol Cristiana Ronalda". But this looks as if his name was Cristian Ronald. I've only seen it once and it was an article on grammar.
(on the other thing, if we considered the real, Portuguese pronunciation, then both words end with -u and it is correct to not change them :D But how many football fans in Poland pronounce it correctly? Almost no one, I guess.)
These are just language-teaching sentences, they do not have to be true at all ;)
I think having statenents that are not always true or sensicle makes it less likely my brain will unsramble the english tiles before i've translated the given sentence, so it works for me.