I can't see any reason that a pharmacy wouldn't want to set up the system for maximum quality, since they should be able to stop recording within minutes after an incident. (preventing overwrite, dropping frames after the fact, or increased compression)
Many of the recordings you see on the TV news are not incident triggered, but taken from the standard always-on recording configuration, which brings you back to the cost of storage space issue. Some systems are programmed to go to higher resolution and frame rates for a certain period to provide better images when something is happening. This requires some kind of an input - either a holdup button or some other type of alarm device integrated with the DVR. In many cases, the counter clerk doesn't have an opportunity to activate these systems without unacceptable danger to himself until after the bandit is out the door.
Motion activated video like you mentioned would help in some circumstances, but in a busy convenience store, this wouldn't use a whole lot less storage space than constant recording because there's nearly always motion present.
If you dive into the issue, you'll find as often as not that the person making the purchasing decision gets personal benefit from minimizing the cost because it helps him or her achieve bonuses, meet budget targets, etc. and poor image quality which doesn't help much in a post incident investigation doesn't affect their performance evaluations or their compensation packages.
Most businesses incent employees based on money saved, without adequate measures for the quality of results. I always caution executives to be careful to consider what folks are likely to do to qualify for the incentives or bonuses they offer, because those things may not exactly coincide with what the folks who set those things up had in mind.