I think a lot depends on the art. In BJJ, most schools do not charge for promotions (although some do). My impression is that most TKD schools do charge for promotions.I think your experience is the exception, not the rule. I could well be wrong, but I have the impression that the vast majority of schools do charge for testing. My own experience is exactly the opposite of yours. I've never trained in a school that didn't charge for testing.
I have no problem with any school charging what they need in order to stay open (or turn a profit, for commercial schools). I just prefer that the costs be included within the monthly membership fee. Firstly, because I don't like hidden fees and I think a consistent monthly charge is easier to budget for (as opposed to the situation the OP finds himself in where he has to come up with $600+ all at once). Secondly, because I think using promotions as a profit center creates perverse incentives. The instructor has a financial interest in promoting students as quickly as possible, while the student has a financial interest in not being promoted. Both of these incentives run counter to the idea that rank should be a reasonably accurate acknowledgment of a practitioners ability/knowledge/skill/accomplishment within a given art.Probably because it's a commercial school, and they have bills to pay.
That said, if promotion requires a special testing session outside of normal class hours, I don't object to a modest extra fee to cover the instructor's time.
Unless you're planning to open a school and teach, then promoting probably isn't necessary. If the money is a concern, don't test.
Depends on the school. Where I teach and train, this would be an accurate assessment. However I understand that many schools determine what material is taught based on the students rank. If the student never promotes, then they don't get to learn the entire art.