Trespassing isn't established until you've been told to leave and don't do so (hence why we have no trespassing signs), the shooter had no right to fire shots at this guy. He should go straight to prison.
If you know you're somewhere you don't belong, you're trespassing. For example, you can't chill in some random backyard until someone comes out to tell you otherwise.
property owners (residential or otherwise) don't really want to ugly-up their properties with "no trespassing" signage that doesn't usually work and really only encourages teens to see what's on the other side of the fence.
He can shoot at people because he was sold a gun and anyone who has a gun can shoot at anybody they decide. What we're actually waiting to learn is "Will this former responsible gun owner get away with shooting at people?".
If the answer is "yes" then other gun owners are going to do the same thing because they want to shoot at people.
If the answer is "no because he didn't have a 'no trespassing' sign" then gun owners are going to buy "no trespassing" signs and then shoot at people, because they want to shoot at people.