Why do you call this a moral issue? Do you have qualms of a moral nature about uprooting a potato bush? Do you cringe from the screams of the wheat as the scythe lays them low? Really, this is nothing but a particularism willing to assign a value to animal life while remaining indifferent to plant life.
Not necessarily, for at least two reasons. First, there are some
vegetarians who do avoid eating root vegetables---along with everything else you can only produce by killing plants. Back in college, for example, I used to know a Japanese couple who practiced one of the stricter denominations of Buddhism. They ate dairy, eggs, seeds and fruits---whatever they could consume without getting anything killed in the process. Both of them appeared healthy and well-nourished to me.
Second, the ethical value of life is not the only reason people might avoid meat; another is the ethical value of preventing needless suffering. We know that animals can suffer, pretty much like we do. Plants, by contrast, lack nervous systems to perceive pain through, so they almost certainly can not. And althought that's no argument against Farmerman's free-range steers, who presumably don't feel too much pain while they live, it is
a valid argument against meat from factory-farmed animals.