It's called ethics.

If going 50 miles makes for better training and allows the student to accomplish the course faster, then great, go for it. If not, then it's the wrong thing to do and you are setting an example of personal gain to the student and everyone in aviation. If you are at DVT, you have a myriad of approaches to choose from without having to go 50 miles, and they do most of their instrument stuff around there at night if I recall (been a while since I flew into PHX at night).

Sometimes there are ways to get that 50 miles on flights that wouldn't normally be, but it has to be ethical and pass the test of being a benefit to the student, not just so they'll get experience, but also cost and efficiency. Often times doing two lessons (one flight to destination, then one flight back) is a total waste, as there's no time for the brain to assimilate any new information and really think about how to do better on the next flight. Although flying once a week is going to incur more cost, flying multiple times a day usually has similar results in my experience in terms of retention of knowledge, skill and progression.