You're assuming that any aliens are super-enlightened vegans. How do you know they won't look at the fact that we eat 90% of the animals on earth and say, "Ah! Clearly these guys are the bosses around here, like us at home."
My theory on approaching alien life has always worked like this:
Suppose you lived in a primitive society, in the ancient past. The king calls you up and holds up a drawing of a lion and says, "I've had reports of these creatures living out in the jungle. I need you to get out there and bring me back a full report on these things. What do they eat? What are they like? Are they dangerous? Etc... etc... etc..."
So, off you go on your lion discovery mission. Are you going to charge headlong into the first pride you come across? Or, are you going to hang back, and observe from a distance?
Even if you were dumb enough to charge into the pride, the king isn't going to get any useful data until someone adopts the second approach. Aliens should probably be taking a similar approach toward us. Eventually, they could work toward some Goodall-style interaction. But, for the initial studies, they would need to be conducted at a distance. It's just good science.