Ha, an interesting situation. It's really just another kind of transquoting [sfc.keio.ac.jp], just like putting an IMG element in my page that has its src= a file on your site. In many respects that's what the web should be all about, and W3C does have public CSS files out there that anyone can use in just this way. But of course those are intended to be public; one should at least ask before linking to someone else's CSS I would think, but if it is, say, a non-profit or educational site doing it I probably wouldn't pester them (except for having bad manners).
Ah, but of course you could do just that, and if you wanted to get back at them it would be very easy: just rename the CSS links on your own pages and leave the CSS file they are linking to in place, but change it however you like. You could make their pages come up with background images, bizarre colors, a tiled pattern that reads "thief" -- just about anything. In fact, forcing someone's site to link to a CSS file under your control would be a good way to wreak havoc in general.