Why? Because we were getting people pissed off that they didn't find out until they went to sign up. We flat haven't had time to do a more complex and friendly/informative solution, but we want to. Forgive us.
Here's the backstory on the websockets decision on an app level: http://andyet.net/blog/2011/nov/14/we-shipped-an-app-that-re...
"The 10,000 people on the planet that care about Web Sockets are not your customers, and while using Web Sockets might get you mentioned on TechCrunch, supporting only Web Sockets is a great way to cut your potential audience in half."
But it all depends on who you're looking to as your market. I'd go as far to say that the group of people who DON'T support web sockets is exponentially bigger than the ones who do...
Pragmatically, though, you probably want to eventually have a fallback mode in this case, with a note telling people that they should probably use a better browser. Team apps are one of those least common denominator problems, after all.