A good point indeed. The whole issue is not just about compassion, but about social order. More precisely, social construction of categories of beings that worthy and unworthy of fair rewards for their work/contributions. Once we accept the notion that some categories are unworthy "human" rewards for their work (cf. horses) - then it is only the matter of merely expanding that notion to other categories, such as unpopular minorities, women, workers....
In the end we have a system of beliefs and legitimations where upper and lower classes are thought to operate on two much different reward systems - the 'economically rational' upper class that will not work until sufficiently rewarded (cf. 7-digit salaries), and the 'economically irrational' lower class that will not work if rewarded too much (cf. no minium wage/ no social safety net policy).
Indeed, the circus and the zoo (esp. the 19th century varieties) are the epiphanies of social order that condemns certain groups to ruthless exploitation.
wojtek