I don't think that is really the argument coming from most vegetarians. It's more like "intentionally choosing the most harmful option is unwholesome kamma, when you have a choice to choose lesser harm".
Wouldn't that depend on the intention behind it?
Perhaps, but even if there is no intention to cause harm, what if the action is simply a product of ignorance or a product of denial? Perhaps one can say that if a person is completely oblivious to the harm being caused, then one can say it's not unwholesome because they are oblivious to the whole situation. However, if a person becomes fully aware of the harm being caused, actually does have a choice in the matter, but chooses to ignore the fact that this harm is being caused and just does whatever, I don't think it can be called completely
blameless anymore because the person is now fully aware of the fact that their choice equates with causing more harm.
If you are fully aware that one choice causes more harm than another, but choose the more harmful one because of some other reason, you could say the intent is not to cause harm but rather simply to enjoy whatever the more harmful choice brings. However, full knowledge and awareness of one choice being more harmful than the other, and choosing the more harmful one for some unrelated reason, still seems to me to bring an element of blamefulness into the picture. Because in order to do that, you have to essentially ignore the fact that you are choosing a more harmful option when you could be choosing a less harmful option. Now if choosing the more harmful option is a matter of real necessity, AKA you actually don't have a choice in the matter, then all of that would not apply.