The rumor that I heard is that it was taken down because they used the "Don't Mess With Texas" trademark without permission and it didn't fall under parody protection.
Apparently (Old article) TXDoT has sent more than 20 cease and desist letters regarding unauthorized use of the phrase.
And if you notice, the custom license plate shirt is gone too.
If you remember, a while back RT was trying to get a custom Texas license plate approved and have all proceeds go to Child's Play. They were very recently denied, and then the video/shirt vanished. I imagine that RT drew just enough attention by trying to get the custom plate to bring the hammer down.
Really? I always thought "Don't Mess with Texas" was public domain since everyone's always saying it all the time and no one is reprimanded for saying it. Then again, I don't live in Texas, so I wouldn't know...
Edit: Er, never mind. I didn't read the article in the earlier post before I spoke.