Why do students with six years of learning English need translating programs?

This week I had a frustrating experience. I teach a freshmen English class at a university other than Iwate University. The students in the class had an assignment due last week(May 11). We had read a story about coincidences and I asked them to write about a coincidence that had happened to them. I asked them to write about 100 words. I never count how many words a student writes but I usually set a word limit to ensure that students write more than a sentence. I was hoping that today (May 18) students would read some of their papers to each other because the topic was interesting. Whenever I give an assignment I am sure to use it somehow in the next class to motivate students to do the assignment. Well, last night, as I was going over the papers I realized that about half of the 20 students in the class had used a translating maching to write their papers. In other words, they had written some kind of essay in Japanese on their computer and had some program translate it all into English. Usually the English from translating programs is completely incomprehensible. I was shocked that so many students would hand me in something that was so ridiculously artificial.
These students are for the most part enthusiastic and I have enjoyed working with them so far. In fact, every week I look forward to the class. This class is not a remedial English class, and I think that every learner in that class has potential in English.
Can you tell me some possible reasons why students who have studied English for 6 years feel the need to use a translating program to write a simple paragraph in English? Also, if you were me, what would you tell the students?
Please respond by making a comment to this post. I will let you know what I told the students after reading your answers.