When is a strait a straits?

As far as I know, there is only one Menai Strait. So why is it nearly always referred to as the Menai Straits? Why is there a restaurant in Menai Bridge called The Straits? Apparently, there is a newspaper in Singapore called The Straits Times, named after - yes, you guessed it - the Singapore Strait, which separates Singapore from Malaysia. Our own Times newspaper referred to "the Bering Straits" when they reported on a madcap idea by the Russians to build a tunnel across to Alaska. It should be, of course, the Bering Strait.

In fact, the only Straits I can think of is Dire, and that's as good a word as any to describe the general use of the word Strait.