It is because he is the one in front of us now.
Compassion is fluid; it is alive in the present and flows toward those who are suffering. It is infinite -- compassion toward Dzhokar Tsarnaev does not diminish the compassion felt for Martin Richard when he is in front of us.
Compassion is not judgment, which makes choices and priorities. Judgment weighs and measures and reasons. Judgment, which we give to the judicial system to exercise finally, will deal firmly with Tsarnaev. It's a whole other thing.
But at every moment, someone has your attention, and in that moment, you will be feeling some emotion: compassion, hatred, indifference, affection. Spiritual liberalism notices that if you build a habit of compassion, you will be happier, healthier, more able to love and receive love.
The world will be better, too.
(I posted this on Facebook on April 20, 2013)