There are two camps on why teachers might disallow calculator:
1) fear of cheating
2) because the teachers want you to do the calculation
When I took my Calc and my engineering courses, my teachers would not allow calculators. They would make sure the numbers are nice, not some crazy number or pi to calculate with. Volts, current, all those numbers are so nice we can do on the paper. I like that because it means the teacher actually put effort to ensure the numbers are not just random, they mean something to the teacher who created the exam (even though they might just change the number next semester).
1) fear of cheating
2) because the teachers want you to do the calculation
When I took my Calc and my engineering courses, my teachers would not allow calculators. They would make sure the numbers are nice, not some crazy number or pi to calculate with. Volts, current, all those numbers are so nice we can do on the paper. I like that because it means the teacher actually put effort to ensure the numbers are not just random, they mean something to the teacher who created the exam (even though they might just change the number next semester).