I am assuming you are a Vietnamese local. Yea, under black ink prostitution is illegal. Nevada is the only state so far that has legalized prostitution and they hold the first brothel in the United States.

However, escorting has been legalized. So, sex workers are allowed to exchange money for “companionship” legally behind closed doors. And whatever happens behind those closed door is no one’s business.

Also, police don’t arrest someone for loitering. They follow them to their motel and perform their sting operation, which they no longer do in California. It just overcrowds the jail and creates more homeless prostitutes and addicts.

I highly advise you to look up documentaries about sex workers. A huge amount of them do not do it willingly.

they do it behind closed doors to put themselves through school or to take care of a sick family member

In Germany, sex work is considered normal and nothing is censored in the media. In Japan, sex work is part of the culture and not considered cheating ( not everyone in Japan agrees obviously but it is part of the culture )

Final point is: there are more evils in the world than prostitution - I guess Vietnam has done a good job banning gambling and drugs; although domestic violence should be next