The idea of a connection between a code against killing and not eating (or pretending to eat) meat, let alone the idea of Superman being a vegetarian, would have seemd ludicrous to the writers and editors of Superman.
Of course, many of these men were well educated and had at least a nodding familiarity with people like Ghandi and Bernard Shaw so who knows? (There was also the example of Hitler.)
For myself, I became a vegetarian out of late-teen self-righteousness and have stayed one for years out of habit. One can imagine Superboy having a similar experience (at least, I like to imagine myself as Superboy, buzzing around my room
)
I think that religion played a part in this oversight. Gandhi was a vegetarian because he was Hindu. I'm a vegetarian for only part of the year, and in the case of Superman's caretakers, they were mostly Jewish. So while Superman probably wouldn't be seen with a giant hambone in his mouth, or eating enough bacon to feed an army, beef would be perfectly acceptable, (though cheeseburgers wouldn't.)
It is an interesting quandary, though. Many meat-eaters support the Endangered Species Act, so the idea of Superman moving and preserving the species of tree is a bad one. However, in my thinking fighting evil also means noncooperation with an evil system...so if Superman really did believe that animal life was sacred, then he should not have cooperated by eating meat. However, that type of Gandhi-ish thinking probably did not enter into the equation in the minds of Superman's Jewish (and later Methodist) writers.
