If you don't think prostitution is immoral per se, why did you answer "it's morally corrupt" to lionking's question? You could have at least said, "[some] people think it's morally corrupt", which would not have caused everyone to think that you held that belief. But in any case, that doesn't answer the question, it just raises a new one: why would anyone think prostitution is morally wrong? Let's say, without appealing to religion since, after all, this is supposed to be a secular country...
I didn't answer, "It's morally corrupt." I answered, "It's morally corrupt?"
Anyhow, your question, "...why would anyone think prostitution is morally wrong?" is much harder. It asks for a mechanism underlying moral judgements, for which, as a materialist, I have no good answer.
For example, I can think of no reliable, scientific method to alter someone's opinion on the matter. Maybe I could do it by changing their experiences with prostitution, changing what their culture tells them about the trade, or maybe a little directed electrical zap somewhere in the temporal lobe. That is quite a string of maybes. And who would fund such a study?
The thread title talks about saving the souls of prostitutes. If I am charitable, and imagine the authorities involved to be honestly mistaken, then their rationale is a moral one. They feel a duty to help others. The others in this case are prostitutes and the help is an attempt to save them from damnation (pardon the hyperbole).
To me, the weak point, the point to attack, is the legality of holding them and essentially forcing them into what I imagine is a program of dubious value.
Trying to justify prostitution doesn't have to come into it at all. The attack is on human rights grounds and the excessive, inappropriate use of governmental authority. The "crime" could just as well be public drunkenness, or littering or speeding - the same mechanism could be used. That's a problem.