I'd also like to to point out that listeria is able to make anyone sick. Not only pregnant women, it can affect any one. So does that mean we should avoid crushers and soft serve and soft cheese for life? (gee I hope not lol)
This is a very good question. The answer is that listeria is more likely to affect someone with a suppressed immune system (and pregnant women fall into this category). And whilst it may make anyone sick, the risk is far higher to an unborn child than to a healthy child or adult.

Personally I steered clear of anything soft serve when I was pregnant, and I never eat soft cheeses anyway (except philly which was safe yay ). I agree with PP that everyone must make up their own mind what risks they are willing to take, but with listeria it is definitely not moderation that makes the difference as it could only take one exposure to have devastating effects. Or you could eat soft serve your entire pregnancy and have no exposure to listeria.