I don't have the impression that people who are against insider trading have a clear definition in their heads, agree with others on a definition, or know what

I don't have the impression that people who are against insider trading have a clear definition in their heads, agree with others on a definition, or know what the law is now in the US (let alone other countries, which can be notably different in law or principles). Why would you ever buy or sell anything unless you thought you knew something other people didn't? But, if that's not insider trading, then what is, and why? When I write "what is", I mean both as a matter of current law, and as a platonic ideal. When big investors talk to company management, what are they doing if not getting inside information? Why is this legal? Is it legal? Should it be legal? Isn't it kind of peculiar if someone that supposedly owns something can't talk to the people managing it?