Can Other Hosted Sites Be Access Through Your Domain/subdomain

I'm using a shared host that provides good service and support, but there's one major issue I discovered recently:

Other sites hosted on their server are accessible through my domains and subdomain. For example:

Account2 on their server can be accessed through:

[url]
[url]

Host says:

"Because everything is Name-based, apache decides what directory is going to be used based upon the name that is provided. However, because the username is valid, it treats it as a vaild URL and redirects it to your site. There is no security issue, as this is just the way apache’s configuration works."

Is it really normal? Does the same happen on all apache servers? I think SE's would treat the other pages as mine, no?