Actually, yes, they were. American cartoons were always intended for kids, it's simply the definition of what is acceptable children's programming has changed substantially over the years.
Edit: let me amend this somewhat; some of the animated shorts that were originally intended to be shown in theaters were more of an all-audiences sort of programming.
And yes, there are a wide variety of animes that were originally intended for the young adult audience that were subsequently acquired and retargeted, so to speak, by domestic distribution companies.
In fact, that's more-or-less how domestic anime distro got started, with shows like "Space Battleship Yamato" being rewritten and subsequently released in the US as "Starblazers". Perhaps most famous of all of that generation of repackaged shows was "Robotech", since it was three distinct series, hacked apart and forced together by good 'ol Uncle Carl.
And yes, most of the older shows that get retargeted are effectively butchered. My personal favorite was how they dealt with the alcohol abuse in Space Battleship Yamato; the bottles became "fresh spring water", or "a new motion-sickness remedy", and so on.
It's important to note, though, that the kids who watch these shows, as a general rule, have no idea that the shows have been butchered from their original format. It's only the pre-existing anime fans who look at the shows and weep the bitter tears of the jilted fanboy.
And it is those fanboys who are perfectly free to choose not to buy the repackaged product, and to write nasty letters to the distro company about how much they suck.