My guess as to why they did this is because someone I know bought retweet.com (a domain that was registered before twitter), trademarked 'retweet' and then told twitter to either pay a licensing fee to use the word or buy them out.
As far as I know twitter bought out retweet.com
Twitter have now learnt their lesson and are trademarking their stuff.
Although I'm pro-copyleft and I'm producing a collaboratively written 'open-source' musical (www.opensourcemusical.com), at this stage I have to agree with Anti-Mike on this one. I agree that using other people's work is a form of giving kudos as it once was 100s of years ago when a composer would be flattered if another composer used his/her melody in their piece.
However if the author expressly wishes that others should copy him/her it seems right (and respectful) to me to honor those wishes.
I'm willing to be talked out of it though. I wonder if you are the same person playing two sides of an argument just to make a dialog interesting. Which is it.
You did a great presentation on Trent Reznor by the way Mike. Hopefully we can prove wrong all those who say 'This only works because he's big'.