It's deeper than that, actually. If you use %category% as part of the permalink structure, then you can change out that section with literally anything in any single post link and still get the single post.
The problem here is the way the rewrite system parses the links. That custom permalink is supposed to be only for single post permalinks. The way it works is that it parses the URL according to that structure, and then breaks it down into pieces.
So, for example, your post here has this URL:
Your structure appears to be "category/year/month/day/postname.html". So it parses that into there:
And so forth. This happens with pretty much every URL, using various rulesets.
Next, it takes a look at what it's got so far, and gives you the most specific thing it can based on that. In your case, it knows the exact postname and date, which is enough to identify the specific post, so it ignores the category. This is why the category can be anything in the URL. Notice that http://www.sierradescents.com/RANDOM/2009/06/13/black-diamond-tents.html works just as well.
This occurs anytime you use category anywhere in the custom permalink string. Why? Because a category can *never* help to identify a specific post. It's never needed. If you know enough to get the specific single post, then you don't need the category. Posts can't share postnames or ID's. If you leave those out, you don't have a specific post to get anymore.
And since this is the single-post permalink string being used here, the category is basically never needed at all. It's only there for show, and because some people think that having category in the permalink helps in some fashion. It doesn't, but it's easier to let them have it than to try to convince them otherwise.
The correct fix is to remove %category% from the possible permalink tags, IMO. But too many people want it, so in it stays. My advice: just don't use it. You don't need it. It only causes difficulty. It's also not specific. Posts can have multiple categories, which one should it use? There's no indication for that use case. It's poorly defined and a bad idea in general.