Ah, the many mysteries of the DH8-400... This may not be the real cause, but I find there is a situation where You do not want to capture a preselected altitude. On an ILS where You capture the glide at eg. 4000ft but the G/A only takes a climb to 3000ft, a leveloff at 3000ft from the glide would be most undesired. SOP dependent, such a situation might or might not occur during non precision approaches as well. This is the only situation I can imagine that takes an alt selection without hitting ALT SEL. Now, keeping in mind the a little less sophisticated -300 that the 400 is burdened with being on a common rating, I would imagine that the reason is somewhere there. Why let the computers bother with deciding whether it is an approach or not when the pilot can just as well tell the box by pushing or not pushing the ALT SEL... This thought may be completely off though.