Athletic events are created by (and for) people who value the sport that's involved. These people might not be prize-winners, but when they participate in an event, they do so with an attitude of seriousness and respect. Charity sports groups often give the impression that their attitude is very different; that isn't a wise thing to do, as the present flame-war indicates.
In subtle and not-so-subtle ways, charity groups tend to set themselves apart from the events they particpate in. It's as if they are creating their own little event-within-an-event. They make it too obvious that, for them, the sport involved is not an end in itself -- it's a gimmick, something that they can latch onto and use for their own purposes. Because their own purposes are assumed to be superior to those of the event organizers, they feel entitled to redefine the event and make their own rules.
How can they not expect that this attitude will offend people? Suppose that a dozen guests at your wedding reception got together and created a separate party of their own within the event -- a party with a theme unrelated to the wedding. Would you find this charming? I'm guessing not. There's no polite way to let people know that you have decided to hijack their event and transform it into something that suits your needs better.
Yes, you can have an ulterior motive for participating in a marathon -- you just can't fling it in people's faces. If you're going to participate in an event, take the event seriously on its own terms. A person who can't run more than a few miles, but signs up to run a marathon, is not taking the event seriously on its own terms. (And in such a case, the solution is to sign up for a more suitable event, not to change the definition of "marathon".)
It is not reasonable to think that you don't have to respect the marathon, but the marathon has to respect you.
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