Very few kids behave better than their parents expect. So why set low expectations?
Realistically, you're the biggest influence on what your kids will think is normal. If you expect them to drink underage or commit minor crimes, they will know it and you can be 99% sure they'll do it. But if you model high standards, you can expect better results.
Some people don't like to hear it, but the biggest reason parents have low expectations for their kids is because they have low expectations for themselves, and they don't want their kids to think they're hypocrites.