It's pretty straight-forward:
Scenario A: a non-US citizen dating a US citizen for an extended period of time, has an intent to get married and live in the US with their spouse, enters the US with a B visa knowing that they will be getting married and AOS'ing = fraud
Scenario B: a non-US citizen visits their long-time boyfriend/girlfriend with a return ticket on a B visa, said boyfriend/girlfriend proposes in the middle of the trip and they decide to get hitched in Vegas then AOS=legal.