Netiquette

I am new to this world of publishing and expect to learn a few things along the way. However, I am pretty sure that most people would have common courtesy when leaving comments, especially another author. I expect and welcome constructive criticism from fellow authors but in the same turn expect they know how to deliver it. I would never leave a negative comment on another authors page for all the world to see. Common courtesy would demand a private message. So why does it happen? Is it stupidity? Is it the threat of another author doing well? Is it an attempt to torpedo success of anyone other than themselves? I'm not sure. However, I do know that everyone should practice proper "netiquette". Grump.