That makes sense - unlike TF2, you can pirate CS and your computer's specs don't need to be that great to play it. So even children can easily get a copy of CS and a computer to play it with.
I'm hoping that the fact that TF2 is free now isn't enough to cause a large enough demographic shift to ruin the online multiplayer experience.
Could you easily pirate CS? From what I remember when you connected to a server it would authenticate a CD key with Valve, and while I'm sure there were servers that had that disabled it was easy to avoid playing on those. Likewise, there were no doubt pools of stolen CD keys, but that was probably pretty hit or miss for those using them.
It also wasn't too hard to find servers with good a "culture" where there were so many well behaved regulars it didn't really matter.
You can't really pirate cs anymore than you can pirate tf2, css also runs on the same engine as tf2 so any of the kids that can get their hands on source can get their hands on tf2 computerwise.
I think that just like with TF2, it depends broadly on what server and community you happen to be in. I've seen incredibly welcoming communities in both games.
I'm hoping that the fact that TF2 is free now isn't enough to cause a large enough demographic shift to ruin the online multiplayer experience.