The question that I usually want to ask is "And how much are you donating? Oh, nothing? Well, I'll put in as much as you are. I'll run too! Yay for curing cancer!"
I get that obviously it must work for the charities otherwise they wouldn't do it - but I still don't get it.
I think it's because there are some differences between charities that aren't always evident. Let's call them "apple", "orange", "banana", "grape", and "almond" charities.
An apple charity uses the thing-a-thon for fund raising and outreach. People who participate pay their own admission fees and often make an additional donation. They are supporting something that frequently has nothing to do with the sports activity and frequently support the apple charity in other ways. Most of the labor comes from volunteers and groups of people that support the apple charity. In my hometown, for example, there's a major running event every spring that benefits the zoo and aquarium. The running event is part of a well planned fund raising and publicity program, and some years it's brought in over a million dollars. It's not the only way the zoo charity brings in money, because the outreach experience is just as big a part of it. But it gives the zoo access to attention and money from people who aren't primarily interested in the zoo, such as runners. Part of the run actually goes through the zoo in the behind-the-scenes area people don't ordinarily get to see. With an apple charity event, you pay to play but there's little emphasis on getting other people to pay.
An orange charity is more interested in promoting the thing-a-thon activity than it is in raising money for some purpose. People who do athletic events or thing-a-thons for orange charities pay fees to participate, but the charity putting on the event is devoted solely to the popularization of the sport. Most of the admission fees go to running the event and paying overhead expenses, because the event *is* the attraction. A small portion may be directed to a health or human services charity but that's more to take advantage of the other charity's name and social capital.
The banana charities are the ones that use sponsorship based thing-a-thons where people hit up their friends and family for per-unit contributions of $x per golf hole or $y per lap. These are either very small local charities where the participants or their families benefit from whatever is raised. Although they tend to be the kind of things many people in the community resent because they don't enjoy being asked for sponsorship or required to solicit money, banana charity thing-a-thons provide a crucial life lesson because they teach people, especially kids, that it's very difficult to get something for nothing, just by sticking their hand out. If they want new jerseys for their soccer team, or if they want that new Little League park, it's just like in the old Britney Spears song: "(they'd) better work, bitch". The labor in question is usually trivial, such as running around in circles or jumping rope, however they provide a better life lesson than, say, the GoFundMe approach where people just stick their hands out. Banana charities help people develop an anti-entitlement attitude. Yes, it's a mathematically inefficient way to raise money. I regard it as part of a character-building process in which the people who benefit from the charitable work have some ownership in the organization: it's part of participating.
The grape charities are highly devoted to fund raising, much like the apple charities, and for them it's all about raising cash beyond the initial participation fee. The responsible ones do encourage people to sponsor themselves before asking anyone else to chip in, because they care about credibility for themselves and for their participants. Not all grape charities are responsible that way, and when a self-absorbed weenie gets involved you get behavior much like what you're complaining about.
The almond charities don't have events of their own, but they sponsor competitors for different events such as, say, the Boston Marathon. The competitor is expected to raise money for the charity in amounts that greatly exceed whatever is spent on him or her. If they fail to do that, they are replaced by a volunteer who can and will. The athletes are expected to act like ambassadors for the charity in question and to actively promote the charity. It is something that generally only appeals to people with a strong emotional connection to the charity.
I go in mostly for apple and almond charities, with orange if and only if I enjoy the activity (I regard them much like a for-profit vendor of services). I did a grape event back in May that involved getting my head shaved bald; it's for an organization I respect that uses its income in an extremely effective and efficient way. For banana groups, I don't get involved in the thing-a-thons anymore or waste time footling around and trying to follow up to make sure everyone gets paid. But I have been known to make a donation if I support the mission.
What I think you're seeing is some obnoxious grape and banana behavior.