If you think these type of people do not exist in your workplace, you are probably mistaken. There are people who will be genuinely offended by things like this (probably due to their own insecurities) and there will be people who are simply looking for a reason to get you into trouble.
And that is exactly why these laws are fundamentally wrong. You suffer legal penalties for what happens entirely inside someone else's head. There is no objective standard.
And I don't think maintaining some joke put into a screensaver for the sake of sticking to the PC police is worth someone losing their job over. There are better battles to fight.
I'm not suggesting that those jokes are effective at fighting this battle. Rather, those who enjoy them are victims of it. I've been in a corporation where everyone's looking over their shoulder for this kind of attack. It's not fun. In smaller companies, people are being silently evaluated for the likelihood of being too easily offended or being lawsuit hungry and are being sent to the resume slush file. Larger companies simply move as much labor as possible off shore to countries without the legal landmines.
This is ridiculous. Understand, in most cases the issue is not the government saying what is and is not permitted.
In fact, the issue is the government NOT saying what is and is not permitted. It's all up to the victim. It's the subjective nature of the law that's the problem. A good lawyer can make "hostile work environment" mean anything he wants, if it can net him and his client some cash.