There's far too many ways to screen to lose money relying and waiting on another provider to say that a client saw them and was nice to them.
No provider alive can truly keep YOU safer than you. Therefore it is absolutely not any other provider's responsibility to provide a reference (that might be an unpopular opinion here and that's okay). BUT it is MOST DEFINITELY a professional thing to do (not to mention it's just good manners to give them if you will easily take them). I do feel for ladies though who feel they are missing out because other ladies refuse to vouch that they've at least met (for whatever that is worth). It's very unfortunate though when someone is happy to take refs but not necessarily give them. Do unto others as you'd have them do unto you, sharing is caring and all that so I give as generously as I hope to receive when it comes to references.
I am happy to share some tips about screening if you'd like....just shoot me an email (not a pm) when you can.