I made a list of those words a while ago (extracted from my dictionary), and they don't really follow an obvious rule, except that they're mostly bisyllabic, only the last vowels are dropping, and those are (almost) all e, o, and ö.
A good part of those, though, are nouns ending on -alom/-elem (which become -almok/-elmek).
The result of both sentences is the same, but there's a subtle difference. Let's try it with a green apple instead, that might make it a bit clearer.
This apple is green. - Ez az alma zöld. - With this I imagine a bunch of different apples, but you single this apple out for its colour. It's special among the bunch of apples.
This is a green apple. - Ez egy zöld alma. - There is not necessarily a bunch of apples, you just state that it exists and one of its qualities. There's nothing special about it.
The same accounts for the famous restaurants. In the sentence given they are not subliminally compared with other restaurants. You just saw in the travel guide that they are well-known and the food here is good. With "These restaurants are famous", on the other hand, you try to make them stand out among other, not-as-famous eating places.