It's more like that there are only about TWO dining programs in the whole country (iDine and Entertainment, now that LeCard seems to be gone as far as I can determine).

These dining programs are too much work for everyone to administer themselves, so dozens and dozens of companies (including all those airlines that offer dining programs under their own names, upromise, and now Diners Club) all contract with iDine to handle it, and iDine simple allows affiliates to put their own name on it. But since it's all iDine underneath, and iDine (not the affiliate which provided the name) is who handles the credit card registration, they obviously know whether your credit card is already registered with ANY iDine affiliate.

Then there's all the other companies that offer a program which clearly isn't iDine, and so far all I've seen (since Diners dropped LeCard) turn out to be Entertainment "under the hood". (So far I've only seen that one rigged up for savings, not mileage earning.)

Simiarly, in "half price hotel" programs, there's about two suppliers, Entertainment and one other.

And the world is full of this. Lots of services, as well as physical products, are "under the hood" available from a lot fewer companies than it seems. For example, a decade or so ago I remember finding out that there were only about SIX major manufacturers of VHS VCRs in the world, and just about all those dozens of brands you saw in stores were one or another of those "under the hood".

I work for a company that makes spa and pool controllers. Not spas, just the controllers. We have quite a few different major brand spa manufacturers buy the controls from us, because spa manufacturers are in the plastic molding business mostly, and most of them (Jacuzzi and one or two others aside) don't find it worthwhile to design and manufacture major components like the controllers or the pumps that go into their molded plastic.