Is it okay to copy somebody else's words and sentences? So I am writing my master's thesis, and am reading a lot of papers. I usually do a lot of 'backtracking' (meaning, read one paper, then look at who they cited and go read those papers).

Doing this, I have found that a lot of authors straight-up copy words and sentences from some of the papers they cite. I don't mean a full paragraph or anything of that sorts, it's almost never more than a sentence, but it does happen a lot.

This is particularly annoying when I read a sentence in one paper and think hmm, that isn't entirely clear and could be expanded upon further, and then I track down the similar concept in another paper to see if there's more info there, but I then find the exact same sentence written there, with no further context provided.

For example, currently I am reading a paper which states 


  The alternating coefficients of the method may mean that in practice, the theorem does not hold.


I'm not quite sure what they mean by this (why are the coefficients alternating? And why does that impact the validity of the theorem?? Does the theorem not take into account the alternating coefficients???), so I look at a paper they cite that talked about this theorem a bit more, and that paper has the exact same sentence (with no further context).

This makes me think that not only was 


the sentence (in the non-original paper) copied straight from the original paper without using quotation marks


but also that


the author of the non-original paper doesn't actually fully understand the meaning of that sentence (just like I don't), and hence, since they don't understand it, they can't write it in their own words, and hence just copied it verbatim. 


Am I reading too much into things, or is this behavior common, and if so, what is the proper etiquette concerning what this is acceptable or not?
As I'm sure you know it isn't correct to just copy things from others. However, there are a couple of additional considerations. 

Something is really plagiarism only if it is copied without citation. Then, the author is claiming the work of others as their own. The safest way to avoid this is to use quotes, as you suggest, but certainly to cite. 

However, if some of the work you find is old, it is also possible that the standards have changed a bit over time and are now a bit more rigid. Citation and being clear about who originated ideas is the important thing, not, specifically, the punctuation. 

But another consideration, especially in fields such as mathematics is that there may be only one way to write some given idea in words. So, in a calculus textbook, for example, the definition of the derivative is nearly identical to every other calculus text on the planet. This sort of thing has actually been discussed in courts, as I understand it. 

But for your own work, be as clear as you can be. Cite anything you use and clearly mark what is being quoted using some reasonable typographical convention. Of course you also need to worry about copyright infringement, but that is a different issue. The sloppiness of others shouldn't bleed over into your own work. 

The question of copying without understanding is a deeper problem. I won't comment on that.