Probably sabra is classed as common because it's a word known to me! However, checking for some empirical backing, I find that the word is not used very frequently. I suspect it was used much more often in the early days of the state of Israel, when most of the Jewish inhabitants were not locally born.
Something similar happened in Australia in the early days of European settlement, when whites born in Australia were known as currency lads and lasses (from the fact that locally produced money was called currency, as opposed to Sterling). These terms have fallen completely out of use in Australia now.
So perhaps the word sabra has become less commonly used over the years. In any case, I agree that it's not so common now, and should be classed as rare.
As for why it is often written in all lower case, it's probably partly because it is derived from the name of a plant - a form of prickly pear - rather than from the name of a nationality. Perhaps also because a key aspect of the meaning is descriptive - born in Israel, not somewhere else. In any case, most dictionaries give it a small S, though at least one mentions a capital S variant. And it is very commonly written as sabra.
As for bolas, it's currently treated as the plural of bola, which is accepted in Chihuahua. It seems bola is another word for bolas. So, the singular can be either bola or bolas, but in either case, the plural is usually bolas. (I say "usually", because some dictionaries allow bolases as an alternate plural of bolas, and this word is also allowed in the puzzle.)
If the term for this weapon in the singular were overwhelmingly bolas, I would be inclined to allow it. But it looks like bola is used quite often, so I don't think I should make a change.
