Sorry, I'm a little confused by your response. Are you saying L4D started out as a mod as well and using the "but" in an unusual way or are you unaware that both CS and TF were originally Quake and HL mods?
"But" should've probably been an "Also". I was only trying to point at the difference between TF, CS and L4D starting as mods and ending up as full games and CTF and Gun Games starting out as mods (even though the CTF concet has existed outside of games long before) and ending up as only game modes in other full games.