I don't know why so many youths need to take the way to be an online fraud. In my country, there also exist such people, but it is not so significant to talk about. Even if 70% can be fraud, police should not arrest them without evidence. As far as I know, an innocent person should not be punished even if 100 criminals need to be spared for it.
Although this isn't an excuse for the police, I guess they've gotten so used to 9 inside every 10 persons being a fraudster that they just feel it's right to arrest anyone... Of course corruption has a huge hand to play in all of this because at the end of the day, all the police are interested in is the money they get for bail.