Lots of images after the read more
As for my justification to this, look at these two cards.

Eagle eyed readers will notice that these cards do the exact same thing. Well, why does this card exist? Easy answer, it gives better mana balancing to decks in commader, with its 99 card deck. Sure, ok, whatever. That's just one card, it's fine to have that as an exception. But take a look at these cards:

Well that's because only having one copy of a gain land leads to having a really inconsistent mana base on a budget for the 99 card commander decks. Maybe a little bit silly to have a functionally identical copy, but c'mon, it's just for lands! Surely there aren't any non-land cards that are exactly the same, right?

This is hardly an outlier, WotC is guilty of doing this up and down. For a full list, see this page. So yeah the one copy of a card per deck rule is completely fucking made up.