As others have pointed out, tethering shouldn't be a concern once tiered takes over. But it is a little strange that some phones, like the Charge, are completely unlocked while others are not. Granted, its easier when you can just load a custom kernel and have permanent root instead of waiting for an exploit, but those exploits always come along anyway, and pretty much everything gets rooted eventually. So I don't really see the point...
I wouldn't count on them being particularly friendly to root at any point though, considering this does run a linux kernel, and is therefore capable of running a number of networking tools that could really cause problems if used by someone with malicious intent. Granted, there hasn't been much of an issue with this yet, but there could be, and carriers don't want to encourage it for that reason. I would honestly expect more companies to follow Virgin's example of using stock Android, and even running more optimized factory kernels before we'll see them encouraging root.