I'm with you. In the circles I run with, it is not assumed that you are treating unless you make it clear via what you say when you issue the invitation! When old friends come back to town they e-mail everyone, and someone sets up a dinner to which all are invited but we obviously don't expect the organizer or the visiting friends to pay for what will likely turn out to be 12 - 15 people! Those who can't fit the dinner into their budget usually make other arrangements to see the friends.
I agree!
I think the key is not to state it as an invitation. Don´t say you´d like to "invite them out to dinner" but rather say "you´d like to meet them for dinner. You´ll be dining at so and so and would like them to join if they care for it".
Tonight we invited some friends to go out to dinner with us (an expensive place at least $100/couple) and we'll be splitting the bill.

)To be safe though, I would probably go with something like, "hey, we're planning to eat at X, you're welcome to join us."
I was also going to invite my old college roomie who she's been dying to meet because he's in the music biz. So, no, we're not going to torture her. At least not on that particular night...
(You could offer to buy them a round of drinks, or pay for dessert.) You know it's a great party when people want to continue hanging out together, & will move to a more relaxed atmosphere conducive to hanging out.