The feedback we get from teachers that use this curriculum is that many like the idea of allowing the students to see the keys. This approach does not work with lower level classes. Those students will often just copy and not really learn.
However, it must be remembered that this curriculum is ADVANCED MATH. Advanced math students are typically curious to see how problems are worked and get instant positive feedback when they can see that they have worked a problem correctly.
In any event, a quick quiz at the beginning of each class on the previous day's homework will generally cure the copying problem.