Because it means . . .

. . . we're sneaky bastards who can't be trusted to keep it in our pants!  They might figure we probably already have a wife or girlfriend anyway, and if we're cheating now, why not later too?

On the other hand, not all providers feel that way.

I think that this response is oftentimes a sort of defense mechanism for a provider, a way to let the providee know that this is just a business transaction and to nip any ideas he might have of asking the provider out on a real date, etc.  Or perhaps it's just a somewhat hypocritcal prejudice.  But I wouldn't sweat it.  I mean, really ask yourself, would you really even want to date a provider in the first place?  Don't get me wrong, this is not a dis on providers at all, some of the more intelligent, interesting women I've ever had the pleasure of meeting have been providers.  But I certainly wouldn't seriously get involved with one, at least not while she was still active.  Friendship, yes (and I have).  But it just wouldn't be worth the stress of the potential dangers these ladies are subjected to.  Ironically, It'd be like being involved with a cop, worrying about their well-being every day on the job (aside from the obvious fidelity issues).

As Papercup says one factor may be trust.  Also some providers view anyone who sees them as a person that can't get it any other way. Also I have also heard providers say that they wouldn't want to be involved with any man who would allow them to continue in this line of work.  Some of the above may say more about the provider than the people that see them.  In any event I do not believe these views are shared by every provider but do reflect the views of some.