Why do muslim men in Thailand have mia noi(small wives)?

Having mia noi may be an acceptable part of Thai culture, but it seems to go against the islamic teaching that ALL wives are equal. Also, the fact that some of these muslim men continually change their mia nois seems a bit like prostitution or dating or 'mutah marriage' which is haram (not allowed) in islam. Many times these men Nikkah (marry) these women and later tell them they are not REAL wives. Women who have married these con-men with the good-intentions of being a wife have essentially been turned into prostitues then and their lives are destroyed. This is very disgraceful! The fact that the mia nois are many times a secret seems also problematic and deceptive. The Qu'ran says that 'wives should be treated with kindness' (An Nisa). This hardly seems kind. In fact it is emotionally abusive for both wife and mia noi.