They weren't taught what counts as plagiarism, how to cite things properly, and what the consequences for plagiarism are. If they weren't taught this knowledge, then sometimes these students who commit plagiarism are less than fully responsible for their acts. On the other hand, a lack of knowledge of plagiarism does not mean that these students are still not responsible for what they have done. They could have taken the initiative to learn about plagiarism on their own, by reading student handbooks or anything on the Internet about it. What teachers should do is get the idea into students' heads that plagiarism will not go unpunished if found in their work and there is zero tolerance for it. .
Students often use plagiarism because of their fear of failure. Some students feel that their writing skills or research skills are too weak to pass a course, so they panic and decide to buy a paper written by someone else. Students also feel ashamed of failing, especially in their family expectations. Students should be told that it is more "honorable" to get a low grade, than to get caught cheating or plagiarizing and ruining their whole academic record. Most students never think about the consequences of getting caught plagiarizing, either because they weren't taught or they think that it's thrilling to do it and not get caught. Students also suffer from poor note taking and summarizing skills. A lot of students get taught in high school that they should memorize facts or definitions, therefore they remember the book definition. It is not their fault that they are writing definitions word for word on essays or tests. Teachers can't count that as cheating, because that is the way that teachers are teaching teens to do. Poor note taking can and often does lead to accidental plagiarism. Unfortunately, accidental plagiarism is not something that can just be ignored. In many schools, accidental plagiarism is dealt with just as serious as intentional plagiarism.