I TA'd several physics courses in grad school, and the only case of plagiarism I was ever forced to deal with was committed by a Chinese student. I taught a lot of engineers who cheated constantly, but only this one student did something so blatant I couldn't look the other way.
The assignment was a short essay. Nothing major... for native English speakers. This student barely spoke a word of English, but was trying to take a Physics course in English. Honestly, that can't be easy. However, what can you do when a student turns in an essay composed of paragraphs ripped directly out of several totally unrelated white-papers? Even if it hadn't been plagiarized this paper would have earned an 'F'. Each paragraph was bizarrely detailed with esoteric knowledge that was both unrelated to the assignment or any of the other paragraphs. As near as I can tell, the student googled random words from the assignment and pasted entire paragraphs into his paper. Reversing the process, I pasted entire paragraphs from the paper into google, in quotation marks, and found the sources this student ripped off, word for word.
As a TA, or even as a prof, you do not make accusations of plagiarism lightly. Most profs will simply warn the student, because making it official involves meetings, paperwork and, in many cases, lawsuits. I went to the prof I was working for and showed him the paper. It was cut and dry, beyond any doubt. We had no choice. Lucky for us, the student dropped the course before we could go any further, so we dropped it.
I'm telling you all this, only so what I say next will have some weight. I've known and worked with many students from China. A degree from a Western university is highly respected in China, and families will spend their entire collective fortune to send a child abroad to study. If they succeed, it's a great job back home and wealth for the whole family. Education is, to Chinese families, a huge investment. Unfortunately, not every student who comes West to study is adequately prepared. They take English courses to prepare prior to coming over, but becoming fluent in English with just a few courses is hard, even if those courses are good, and many aren't.
So, put yourself in the shoes of a Chinese student. You've prepared as best you could, but you're in a foreign country. You don't speak the language. You don't have any friends. You're studying something that would be challenging if it were taught in your own language, but you're studying it in a language you can't really understand. You're getting failing marks. Your family bet everything on you coming home with a degree. What the hell are you going to do? It should surprise nobody that these students cheat when they feel the noose tightening around their necks.
The truly horrible thing about this is that a lot of Universities, including the one where this happened, have free programs that offer support for students who need help with their English. They have staff and volunteers who speak Chinese, and many other languages, who will translate for and teach students in this kind of situation. However, many of these students are so hopelessly out of the loop that they don't even know where to go to get help.
So, there you have it. Plagiarism happens, sometimes out of desperation. I'm not saying it's excusable, but it is understandable.