It always makes me laugh when people say this as they haven't a clue what others get up to.
Whilst it's true that you can't be sure what the prossie gets up to in her own time (and to think she has never barebacked anybody is obviously ridiculous) the argument holds that a girl who doesn't overtly state that she does bareback is likely to be a less risky punt than one that does.
Should the condom break without my knowing I'd rather it happen with somebody who doesn't offer bareback. A prossie who only barebacks her boyfriend, or on the occasional one night stand or even just with regulars is less likely to have something nasty than somebody who indiscriminately barebacks (KKK, I'm looking at you).
It's all about risk management and how much risk you are prepared to take. I personally will punt with a girl who offers bareback but she will likely need to be fitter/cheaper/nearer etc than the alternative and of course I always use protection.
Also I think you can infer a girl who offers bareback is likely to be less than meticulous in other areas of her life and may be more likely to be unhygienic or live in a shithole. The flip side to that is she will probably me more dirty, in a good way, than those sterile fucks we've all had where oral is protected etc etc.
A sort of related joke: What do you do if your girlfriend tells you she has HIV? Act surprised.