I understand other races are named by their distances. No one is confused or offended by someone saying 5k or 10k, but also no one familiar with running would call those races a "5k marathon" or "10k marathon".
I also understand that a half Ironman is also called a 70.3 Ironman. The difference there is that "Ironman" is a trademark held by the World Triathlon Corporation. Other races can't call themselves a half ironman or full ironman so they often use the distance (70.3 or 140.6) and just like we use Kleenex as a generic term, people sometimes use the term "70.3 ironman" to describe a half Ironman distance race. However, in my experience the preference seems to be "70.3" or "half ironman".
I tried to find an explanation for this naming convention. Wikipedia doesn't know what a 13.1 marathon is and the first several pages of a google search all point to the Michelob race. At first I thought it was comical to see "how far is a 13.1 marathon" but then I realized I had already asked and answered that question by visiting the site above for the Michelob series. Not surprisingly, it is 13.1 miles.
I can only assume that the goal is to make this race more approachable. Not everyone knows how far a marathon is. Most people have never run one. They know it's a long race. They might know you get medals and other "swag" when you complete one. They might even know you aren't required to run the whole time. Calling a race a "13.1 marathon" might make running and racing appealing to more people which I am all for. It still makes my skin crawl.
I'm a very typical runner in many ways. I started in 2009 and did a "couch to 5k" program pounding away on a gym treadmill wearing cotton gym shorts and heavily discounted Nike trainers that were too small. My wife and I ran our first 5k in a relatively small race but we got to wear bibs with numbers on them and had our time recorded electronically and had people we didn't know cheering us on even though we were nowhere near the front of the pack. I finished that race barely in the front half but I was hooked.
As for my commitment to running, I might be a little more aggressive than some. After that first 5k, we moved straight on to training for a full marathon and threw in a 15k for good measure. After my first marathon in January 2011, I focused on 5ks and half marathons for a while. By the time I ran my second full marathon in 2013 I had moved from the middle of the pack in that first 5k to the 89th percentile in the Marine Corps Marathon.
Why am I sharing all of this information? Am I just bragging? Maybe I am bragging a little, but that is because I have earned it. I'm not bragging about finishing ahead of about 89.4% of people who ran with me in DC. I am bragging that I ran that race 45 min and 22 sec faster than my previous race at that distance. I am bragging that I dropped my average pace per mile from 10:12 to 8:28.
You see, most of us who do go out and run every week and race often aren't racing against each other any more than someone who just does one race to cross it off of their bucket list. We are racing against ourselves. The marathon is the "premier" distance for people who want to improve their running. It is the gold standard yardstick for measuring endurance running performance. That's still not why I find the term "13.1 marathon" offensive.
The real crux of my contempt is this: the body starts to "break down" after covering 20 miles. I'm not going to try explain the physiology, but I will tell you this: even now at my lowest level of fitness and training in years, I could still run 13.1 miles without stopping. It will be slow, but I could finish. I could not run 26.2. The marathon is special. So let's not water it down by trying to turn it into Kleenex.