It is because most people hear the term "hack" for the first time in a context like "Someone hacked my computer", meaning the term "hacking" is associated with

It is because most people hear the term "hack" for the first time in a context like "Someone hacked my computer", meaning the term "hacking" is associated with say breaching security, which may be illegal in some jurisdictions. So he qualifies it with "ethical" meaning don't use those techniques (whatever he claims) for unethical acts.