The point is that the quality of compassion is nurtured by a vegetarian outlook - you may be compassionate to a greater or lesser extent but if you adopt practices that cultivate the quality of compassion then it stands to increase - an obvious way to cultivate a quality is to be attentive in all circumstances where it appears - like for instance if you just "turn off" in reagards to animal slaughter you miss out on an opportunity to operate on a more compassionate level of performance.
You may appear compassionate by supporting the local economy, but you could also be a drug dealer by the same logic - in other words wisdom consists of making the distinction between long term and short term benefit