nope, it's more like how humans used to eat horses but dont view it as something you should do anymore and it's still a fresh take... so they aren't finding it morally objectionable, just legally repercussionable to do
FYI: I haven't read past this chapter yet.
However, as mentioned, the red alien (demon?) states it clearly; It is
not illegal to eat humans. What he says is that it's a social taboo. The only reason an alien would care about not breaking a taboo is if there is a moral standard against eating humans. As in, other aliens would look down on you for it and potentially mistreat you. Taboos only exist if a wider culture considers an action repulsive or otherwise wrong, even if the law has no say in it.
Humans clearly aren't protected. If they were, poaching them would be illegal. So it's in that gray area of, "well, you CAN eat them, but you'd be wrong for doing so." I believe it's also significant that the child wakes up on a train with a bunch of cows, showing us that normal livestock are still being shipped around in this world.
The bandits are clearly just starving and desperate and are seemingly living in an environment where a "black market" of human trading exists.