The problem is yr vocabulary. You are not using the term "legalise" in the right way. Prostitution is allowed in Singapore - meaning S'pore does not outlaw it. So paying for sex and selling sex is per se, NOT criminal or an offence, and police cannot haul you to court. When you say prostitution is illegal, do you mean the buyer and the seller are committing a crime? Because that's what it means.
Locals -Singapore girls, PR holders - can sleep with men for money and still is no crime. Yr girlfren asked you for money after sex, or you gave her money after sex is no crime. She may be busted for not paying taxes tho, if she becomes a high roller, but that's different.
Singapore Ice as you said sold sex while working as masseuse and ktv lunge waitress. Now she was busted --not for the paid sex part --but for doing it in an establishment that is not licensed as a brothel and specifically has prohibited sex between patron and staff, never mind paid or not. If she had been smarter, she shld have arranged to book into a hotel and fuck till kingdom comes and still neither she nor her partner can be busted --for prostitution, even if she was sleeping on money bills.
Same with foreigners here on student pass, visit pass. They flout the law for which their visits/passes were allowed. If you pick up one of them and fuck in a hotel or in yr house or car, the police got no case. But if they stand on the streets to solicit, then they commit an offence for soliciting.
The police has to tread a fine line between personal freedom/privacy and rampant uncontrolled sex trades. That's why they have GL, KS and DR --to keep it open and regulated. That is the word.
"Legalized prostitution does not necessary meant prostitution is not an offence, if it's not approved, it's illegal, it's an offence."
I think you are really confused above and below.
"They can legalize prostitution, they can also ban illegal prostitution. "
Hope to have cleared the air.