Well, how come there is no murder-culture? (answer at the bottom of the post)
We (in the US) casualize murder through television. We glorify serial-murderers, which often have rock-star style attention, groupies, and fame. Kids often joke about killing one another. Kids are happy to tell you when "they killed the bad guy". "Killing it" is a positive phrase of accomplishment. Paintball is a somewhat popular sport, which is nothing but people acting as if they were on-duty soldiers ready for killing, we kill animals for sport, and we give medals to soldiers who have killed 'the best'.
There is definitely a huge problem with murder here..
Why there is no murder-culture: What rape-culture really is is the problem that we as a society have with placing the blame of a rape on the rape victim and not the perpetrator. Rape-culture is when a victim is criticized for their location and clothing; common due to the misconceptions behind how a rape occurs, and because of the tendency people have to be unable to place blame on an unknown quantity. A terrible tragedy that occurs too often. This, of course, cannot occur with a murder; both due to the social stigma of criticizing the dead, and simply for the fact that there isn't any longer a person to blame other than the perpetrator.
It has nothing to do with the background use or casualization of a phrase, and that's the (wrong) kind of thinking that actually will hinder free speech.
There is murder culture, to an extent -- the main difference is that the while (as you yourself describe) there are some social context similarities with murder culture to rape culture, those social manifestations don't as frequently connect to a deeply entrenched power imbalance and license abuses across it the way rape culture does with sexism. So murder culture, while it exists, isn't quite the same kind of problem as rape culture.
OTOH, there are a number of what might be called murder culture does have a problematic interaction with various entrenched power imbalances (racism, most notably, but also sexism and some others), but while the association is very tight between sexism and rape culture by nature (that is, rape imagery inherently invokes the same divide at issue in sexism), murder culture only overlaps with rather than tightly maps to the divide at issue in racism or any other particular power imbalance.
You, uh, might want to look at what's happening in Ferguson, MO right now.
I wish I understood your perspective on this a little better -- reading through your comment history, you obviously have a pretty nuanced view on a lot of things, but from my standpoint you frequently conflate the right to free speech with a right to consequence-free speech (note that even in the tweet you posted, where the author hyperbolically claims to want to "stifle free speech", he includes the caveat that he doesn't want people to be "comfortable" joking about rape. I think that's an entirely reasonable sentiment).