The problem is, to the victims, these people are NOT strangers. They talk every day, they email every day, flowers and small gifts will undoubtedly be sent, photos, love poems, constant messages etc. The victims end up believing that they are in a full time serious relationship with this person. They become all consumed with them. They fall in love. They are not thinking because their will is completely overborne. They trust this person and have every confidence in a happy future together. They feel loved, happy, confident, respected, cherished, listened to. That niggling little doubt in the back of their mind is so easily dismissed when he or she calls again and gives words or gestures of reassurance - which in turn leaves the victim feeling guilty about their thoughts!
They start off with small amounts and then it ramps up. The victim (instead of calling a halt) goes through all sorts of emotions and pays up more money because that is the only way they can see to protect their future and the investment they have already made. The psychological tricks these people use to then extract more money are similar to the psychological tricks a perpetrator of abuse uses in order to ensure his or her victim does not leave.
It is classic boiling frog. You put a frog in a pan of water and turn on the heat. By the time the frog realises it is boiling, it is too late.
Yes, I see why some might call it madness or a lack of mindfulness, off their trolley etc. But that is the point - they have been completely overcome and they have been persuaded to believe.