1st site: https://www.firstsite.com:12345/
2nd site: https://www.secondsite.com:24328/
3nd site: https://www.thirdsite.com:37712/
Normally this isn't a problem, except that some browsers nowadays default to requesting the HTTPS version first. That means if you're serving an HTTP site and an unrelated HTTPS site, and a user accesses the HTTP site via an HTTPS connection, there's a good chance you'll end up serving the unrelated HTTPS site plus an SSL warning instead, because that's how Apache does things.
How is that Apache HTTPD's problem? Nginx would do the same thing. When you don't define a ssl version of site.com for ip:443, you get whatever the default cert/vhost is for that ip:443. What would you have a webserver do differently?
Sure there are other options but I'd rather not have accounts all over the place, and I like Linode.
Linode's problem with $10 accounts is not with customers like you who are paying them decent money otherwise - it's with customers who would sign up for $10, break things and expect $50 a month worth of support on an ongoing basis.
Most customers looking for a cheap VPS (to learn on, to experiment etc) are not going to be profitable unless you provide virtually no support, so by slicing off that part of the market they can offer a uniform level of service without compromising their profitability.
How about not revert to a "default"? Just serve a redirect to the HTTP version if a HTTPS is not available for that vhost.