I didn't see any explanation of this in the notes section.
"étterem" is singular.
Normal rules to make it plural, would be to add "-ek" ... so "étteremek".
But this is an exception to that rule. Some words drop the last vowel in the root, so "éttermek".
Vowel dropping is quite common in plural form but I don't know the rules off the top of my head. I think they all end in something similar like -rem.
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).
Yes, transformations in the word root like this should definitely be added to the notes.
Is there any reason why "These restaurants are very famous" would be incorrect?
I wrote the same answer as you did and was also marked wrong, I reported it.
It is wrong, though. "These restaurants are very famous" would translate to "Ezek az éttermek nagyon híresek." Different grammar.
I see, I thought it was a matter of word order.
BUT, though I get the grammatical difference, isn´t the meaning of both sentences the same? Don´t they convey the same message?
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.