It's understandable to frame it in this way, and there are legitimate predators who take advantage of people in those environments. The common case is more prosaic: addicts are often codependent and will seek romantic companionship amongst each other, especially in close personal environments like that. Starting a relationship early on in the recovery process is a common (yet tempting) mistake and conventional wisdom warns against it, which is why the phrase exists.

[I spent a few years in the orbit of 12 step programs in my youth]