Wasn’t always the case. Bambi’s mother died, so did loads of people on the original American release of Gachaman, (Battle of the Planets, I think) though mostly not onscreen. Still cut, (most humorously the parts with swan-girl killing people with her bladed yoyo, then doing tricks with it) there was death and blowing up buildings and cities had consequences, even without showing blood.
It was in the 80s that this became more or less mandatory. (Note later, death-free re-release of the same show as G-Force) Possibly as sort of a pre-emptive backlash to the less cartoony kidshows that were showing up in the mid-80s. (He-Man, Robotech, GI Joe, Transformers and the like)
More likely, though it was a result of a combination of a typical 80’s obsession with perception and image. The companies, particularly the TV stations and the sponsors that made the shows wanted to look purer than pure, so anything even vaguely negative or antisocial got cut short. Wasn’t the mid to late 80s the era of constant ‘viewer boycotting’ of stations that offended them? Blaming the media for all social wrongs was a popular hobby at the time, even moreso than today, and people who are involved with kids have always been paranoid about their reputations. Thus, content-free kid shows, and PG-13.
It’s not really that different from the way the Comics Code got there years earlier. Self censorship can be really hard habit to break, especially when the people enforcing it are distant from, and not answerable to, the people actually making the shows.
As Carter the Great mentioned, much of the anime shown on American TV was meant for adolescents or young adults. A fair amount of it, including much of the weirder stuff, is meant largely for geeks specifically, who have notably more exotic tastes in entertainment than ‘mainstream’ audiences. Until recently, though, anything that was animated was pretty much by definition for kids or a sitcom, though, so of course it’s going to look weird when they start kissing with tongue.
Again, the ‘comic code’ is a fair analogy. The average age of comic book readers is something like 25, and has been for years. A demographic that’s traditionally fond of decapitations and nipples, though hopefully not at the same time. But because that’s not the common understanding of what ‘funny books’ are, (and also because the comics industry did a fairly poor job of distinguishing between what books were for whom) the whole industry sort of froze in a confused ‘I’m no longer a boy, but not yet a man’ state, not really able to market effectively to either demographic.
Though that has little or no bearing on the question at hand. I don’t even know why I brought it up. Video Games are caught in a similar bind at the moment, though they make a lot more money, so the industry has a lot more muscle. ANYway . . .
The weird thing is, there ARE animated ‘kid shows’ in Japan, but instead of bringing THAT stuff over, until very recently they’d bring over the young adult shows and try to stuff them into completely inappropriate markets. I’m not sure if they were trying to capture both the youth and college markets, (which worked for a while) or if they didn’t want to compete directly with the domestic shows, or both.
Possibly this was simply an extension to turning the pre-movie cartoons of the 50’s, intended for mixed audiences with a fair amount of adult, and frequent sexual humor, into the standard entertainment for kids of the 70s and 80s.
If nothing else, though, it did clearly point out just how ritualized and goofy cartoon standards were. I’ve always found it a bit odd that violence and sadism (Tom&Jerry?) are fine, as long as you separate them completely from any kind of consequences.
Tranzor Z: Viewed objectively, breast-launching is only moderately weirder than shooting off hands or (in the case of one memorable robot from the same show) heads. Besides, Getter-Robo was put together by Go Nagai, so you have to expect a little of this kind of thing. 
If you’re really curious about non-Japanese fucked up popular culture, though, do a search for ‘Fumetti Comics.’ I’m sure you’ll find more than two. Don’t say I didn’t warn you.
Sorry if none of this made sense. I haven’t slept.
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‘Something else created by Satan. Japanimation!’
