The early ones did have serious programmers at them.
My first memory of one is Yahoo using it to get people to use its API in London, about 10 years ago. That was when a web API with major data behind it was an important new thing. The openness was quite radical, and linked somewhat to the open source movement.
We (ScraperWiki, Rewired State...) did a bunch of hack days for mainly philanthropic purposes - democracy, journalism and so on. More recently DataKind does that even better.
Now though it has become so mainstream, there are hackdays everywhere. They're being driven even more by marketing, rather than coding. The original purpose feels lost.
Well, it depends on hackaton. Most of the hackatons now are commercial events, designed either to milk participants for free work or promote some third party services.
I remeber when hackatons were made by programmers for programmers, when they were about hacking on actually cool projects and having fun. Those hackatons had some serious devs participating, because hell, even if you're doing this professionally, you need to take a break every now and then and do something just for fun.