>why they don't implement a simple script to stop typing beyond the limit is beyond me...

I'm thinking that once you've written out the text you seek some place to post it and so this sort of thing increases interactions on the site, after all you're already there - you make it into a message to several friends or start a page about it or whatever. Alternatively people put it on a blog and then link their blog into their profile in someway, again increasing the information available on that person, strengthening interactions and widening the link graph.

Just my initial thought, no inside info.