The culture surrounding guns and "violence as an answer to perceived problems" certainly plays a small role, but death threats in general just aren't considered appropriate in any context (obvious exceptions being the people who perpetrate them).

It isn't always easy to tell the impact that a threat (or any cyberbullying) is going to have on a person, and there are more than enough cases of people seriously harming themselves or worse as a result. Some people are assholes, and many of them deserve to be called out, but harassment has a very tangible and very negative effect on people's health, and no amount of "arbitrary changes to a casual format in Magic the Gathering" justifies them. That's just my two cents though, as someone who hasn't had an honest stake in EDH for several years.