In reading the articles it seemed there is no reason as to why these countries are seeking to make the buying or selling of sex illegal. That is of course, outside the alleged increase of "sex slaves". Notably, these people are said to come from poor economic backgrounds and they see sex as a viable option for them to support themselves out of their country. Their poor economic background does not necessitate that they are trafficked. Rather, it demonstrates this profession is something they are able to do and the compensation appropriate for their time.
Also, it is worth mentioning, rather questioning, what does the government gain by making buying or selling of sex illegal? Is there a substitute product that the buerocrats have their hand in? Are they earning a profit from porn or strip clubs that may be more lucrative?