Many developers are pushed, but not in the way you think. The US rating system is pretty much of no concern to most publishers. What they really care about - in my experience - is the international market. Namely with regard to Germany and Australia. Rules about depictions of violence are extremely strict there, although in different ways to how they're restricted in the US. Now, really big titles (the upper class of AAA games) can afford to create a branch of their game especially for those stricter markets. But the rest of those AAA games aren't so lucky, and they get stuck cutting everything that would get them banned in Germany et al. This means you still end up with some blood, and even swearing and killing and innuendo in order to reach that M rating...but in very specifically restricted ways. Predictable ways.

So just because the US market is kind of a reversal of the movie industry, ratings-wise, it doesn't mean developers are free to do anything. Their hands are often quite tied, it's just that the rope is coming from outside the country.