So my question is following: when trademark should be enforced and when not? If a company X is an internet consulting company and somebody has a blog talking about internet using the same name, is that trademark infringement? Does Tumblr (or any blog operator) need to contact user to explain if they get Demand Letter from lawyer representing the trademark owner and Tumblr's legal department determines it is valid?
I'm not trying to defend anybody but just want to understand how trademark can be enforced.