I get what you are saying, and said I figured that was the reason. I am just wondering if rD has ever specifically indicated their thinking? I have been to many 'fun runs' with far lower fees that manage to chip time.
Based on personal experience with family, I think it lessens the sense of accomplishment for some new or first time runners for rD to say that the slow general pace of the finishers in not worth timing. They still finished. No one has to stop for pictures, those that want to run or walk it for time can. Those who don't care about time, are free to go as slow as they'd like. Particularly for weekends like this where the 5k is not part of a challenge, they could easily increase the pace per mile required for the 5k and eliminate concerns of those not meeting the 16mm. I am sure they know what there real limit is for when everyone needs to be done, and they could set the bar accordingly. I still have not heard a first person story of someone getting swept from the 5k, so that limit would have to be pretty high.
It took my six year old daughter, mother and I over an hour last year with pictures, and goodness know any other time I'd be pretty unhappy with a hour long 5k. I knew going in there was no official time. But it doesn't mean I have to agree with the decision. It was my mom's and daughter's first 5k, and officially, they never did it. Now, we are all signed up again for 2016, so clearly I am not that upset about it, but, I do still question their thinking.