Hopefully, most teachers are in the profession to educate and help the students. They can choose to be paid over a 12 month span, or, the 9 months they generally work. Many seek other jobs for the Summer. If they can stay in their same school district and get paid for the 3 months they are normally off, it's a Win for them, and a Win for the students.
The teachers I worked with did it for the money and because they would rather teach than stay at home. I taught Reading in college. The students had up to three semesters to learn what they should have done in 12 years. If they didn't pass, they lost their chance for a career and a better life for them and their families. It was the hardest job in my life & the most wonderful gift I could give anyone. I usually wanted time with my own children, but if more than two students begged me to teach in the summer, I always did it. They knew it was a difficult subject and summer is harder than fall and spring classes because they had less time to learn the same material. I loved them as a family and you make sacrifices for those you love. I had an accident & have had 10 surgeries in the last 14 months, but I still have students that stay in touch through Facebook. I would give almost anything to be able to go back and teach. Right now I am working toward trying to be a volunteer tutor. Does this answer your question? Sorry it was so long. Being put to sleep so many times has hurt my memory with spelling. I am working on it so please forgive any mistakes.