Earlier quoted context omitted.
I still can't send text messages over 160 characters to anyone not on my carrier (Verizon) so I'm not sure where you're getting that figure from.
http://en.wikipedia.org/wiki/Concatenated_SMS Perhaps this is another anomaly that Americans have to put up with from their carriers — not being one I wouldn't know.
Just yesterday I even noticed Android's core SMS app is even nice enough to tell you when the message is overflowing into a second SMS, so you know that you and the recipient will get billed for more than one SMS. Which brings me to another idea:
Perhaps the billing issue is more of a concern as well -- receiving concatenated messages certainly would cost more for (American) users not on unlimited SMS plans, and presumably would cost more for Twitter to send out through their gateway?