Realistically..it's not a "big deal"...just an annoyance. But it does, in my book, indicate an improperly configured web-server. IF they don't have a valid certificate for https; then they should disable https entirely so things like this don't happen. But the webserver isn't even remotely configured properly since clicking through the error on https just nets you this:
Not Found
The requested URL /php.fcgi/index.php was not found on this server.
So this tells me that even if they had a valid cert....the webserver isn't even set up to serve pages from the proper location. Furthermore....the cert they do have is self-signed...indicating they're using whatever default configuration the webhost set up for them.
Normally a server is setup to redirect to https if it's available....it's actually a few options you set in Apache when you configure your https domains in the first place..since technically they are served over a different port and require their own configurations. I have about six websites and I set each one up for https because "that's the way the browsers run" and it didn't cost me anything. Hell, I have some web applications that *demand* https:// support and won't respond unless you come in through that method. My ZNC bouncer for example, has a web-interface available on a special port using https:, but it will ignore you if you try http.
But more and more browsers are starting to force https by default, even if the server doesn't indicate it supports it. Older browsers will still pay attention to the server and *should* roll back to http if it doesn't support https. https support has been "standard" for quite some time now.
Ideally, if they don't want to support it, they should disable support for it entirely in the webserver rather than relying on a broken configuration and users over-riding the https for http. I can only suspect browsers will stop allowing you to do this in the future...meaning that unless the problem is solved in a proper way, a lot of people will be unable to access the site at all. Chrome did not want me accessing AK today...at all...I had to over-ride some settings to make it not default to https.