Maybe they think the rules don't apply to them, or maybe no one else has conducted any type of screening on them. It's a classic case of thinking with the small head.

that information along with references is what I would prefer to provide.  It is usually sufficient, but some providers ask for more, commonly a last name.  I'm always a bit hesitant because my first and last names are uncommon enough that it is close to unique.  On occasion, the unfortunate can happen and one's name can end up in the wrong hands as we have heard of recently in Phoenix.  John Smith with a disposable phone is protected by his common name, I would not be.

I still provide it when I feel comfortable with a provide, but I prefer not to.  When I was just starting I more commonly did have to provide full name and work info.  

If someone thinks they can get by with just a first name and number, you're doing just fine by explaining to them that you require more info and hanging up if they want to argue.  Whatever you require, you deserve respect for your choice.  If someone doesn't feel comfortable with what you require, they should see someone else.