He's entirely justified there. That stock email could have cost him a lot of money and hassle if his ISP had been a little less clueful. Sending it was grossly irresponsible.
Sending legal threats like this, whose validity depends entirely on the context, with no idea of the context, is grossly irresponsible.
He did not sink to their level. If he sent them and their ISP baseless legal threats that might result in a temporary loss of internet access, that would be sinking to their level. If he tried to trick their domain registrar into transferring their domain to him, that would be sinking to their level. If he tracked down the families of the legal staff and threatened them in their homes, that would be sinking to their level. All he did was write some bad words in his blog.
