What’s the deal with WordPress.com Akismet API keys?

OK, so I set up a new blog for a mate on my server, using a shiny new WP install of course, then I go to the plugins page to activate the much vaunted Akismet plugin, but find out that I have to get an API key at WordPress.com before I can use it…

So what gives?

I’ve got no problem at all signing up for a WP.com account, but surely that’s wasteful since once I’ve signed up, I’m just going to use it on my box, and leave the WP.com account to stagnate… why not just let folks get their API key from WP.com?

I’m not sure if I have the whole story – perhaps there’s an option that says “I have my blog somewhere else” but still allows you to get your API key – so I could be wrong, but this kinda doesn’t make sense?

What do you think?