Common argument is that such storylines would be disrespectful to real people struggling with these problems. For example, fans want Wonder Woman as an icon for feminism/LGBT, but they don't want storylines where she was active in early 20th century and participated in these movements that gave women right to vote and so on. So you know, idea probably is that if Jon actually did something to fight climate change then activists (and people in general who care about this subject) would be offended. DC would piss off audience that they supposedly want to attract.
Lets take another example, Ennis thinks that Captain America is a disgrace to real WW2 soldiers. Common counter argument is that Captain America was actually popular with the soldiers so Ennis is full of ****. So question is, how do we know that people interested in feminism/LGBT/climate change would be offended if WW or Jon actually did something? DC probably thinks its better to be safe than sorry and well, thats why we are where we are.