Sometimes moral outrage is appropriate. It would be strange for someone of conscience to dine with a bigot. But a vegetarian is expected to eat, work, and live with those who eat meat. Given that some vegetarians consider non-human animals to be persons, this expectation is out of step with standard views about what conscience requires.
Moral outrage is sometimes required by conscience and sometimes not. I'll take that for granted. Under what conditions is outrage required by conscience? Presumably, it has something to do with the behavior that is the object of the outrage. As a first approximation, the behavior must be sufficiently heinous. Moral outrage over spilled milk will not do.
But a moment's reflection makes clear that this is not right. If an action is heinous, but I do not believe it to be, I remain in good conscience without being outraged. Similarly, if an action is not heinous, but I believe it to be, it seems that I cannot remain in good conscience without being outraged. So, a second approximation: one's conscience requires outrage over some behavior if and only if one believes that behavior to be heinous.
So, where does that leave our ethical vegetarian? They believe that the consumption of meat is heinous, but they are not outraged by it. Can they, in good conscience, remain unmoved by the sight of meat consumption? Two excuses come to mind: first, perhaps outrage is required only if a behavior is viewed as heinous by the wider community and, second, perhaps it is not meat eating that is viewed as heinous but slaughter.
The first excuse is a non-starter. An abolitionist of good conscience living in the antebellum South must be outraged by slavery, even though most everyone in their community takes it to be acceptable.
The second excuse might help, but only so far. Insofar as someone considers the act of eating meat itself to be heinous, it is plausible to think that their conscience requires outrage. Similarly, insofar as they consider the mistreatment and slaughter of non-human animals to be heinous but that consumption of non-human animal products to be merely impermissible, it is plausible to think that their conscience does not require outrage. Let's grant this.
This only pushes the issue back a step, however. I live in Nebraska and work for UNL. Some of the students I am in regular contact with are working toward careers in feedlot management. It still seems like it would be inappropriate for me to be outraged by their behavior (they're good kids!), but it cannot be because I don't take their behavior to be heinous.
Perhaps the injunction against outrage in this case is not a moral norm, but rather a social norm. It is certainly possible for a social norm to conflict with a moral norm. But that leads to the uncomfortable conclusion that I am not nearly outraged enough.