Because that's not how he's supposed to act, even that Superman has been Superman for a while but acts like he's on his first week. He's not Batman in a red cape. He had no business doing that in either of those situations, he wasn't in danger there. And given how dangerous his powers are it'd be incredibly stupid for him to be a super-hero if that encounter with Lex was all it took to nearly burn a person alive. When did Post-Crisis Superman ever do things like that?
Only by people who don't know the character. People used to think Captain America was a joke, too. The only thing that separates Superman from Cap is bad writing. That's not being typecast that's how he's been written for the majority of his time when he's owned by DC Comics. What DC has trouble with is writing him so he's not a caricature, especially in live action. There's more options than boy scout parody and nearly burning someone's face off with his heat vision.Superman has been typecasted and boxed into so called stereotypical boyscout roll for so long that we get these kinds of reaction from something very trivial.This is how people see superman as.
Though i would admit this pretty funny.