This is not true, and people should be very careful not to make such strident statements when they have no basis in fact.
There are a select number of games for which one can purchase a copy in a brick-and-mortar store and activate it on Steam. You can find that list here:
https://support.steampowered.com/kb_article.php?ref=7480-wusf-3601. As you can deduce, there are tons of games that are available for purchase on Steam but not on that list.
This isn't to say that you cannot buy Civ5 in a store and activate it on Steam. I agree that, if the game has Steamworks and will require a download of Steam to play, it's a fair bet to say that you can just use the key. But it is to say that you should research the question before you make wildly inaccurate statements that could cost someone $50.