[WS:] This might be the case, but I also have a somewhat different interpretation: expectation of one's 'social affinity' group (or "class fraction' as Bourdieu would say.) For the rich, philanthropic giving is an expression of social status - as documented by Galaskiewicz in "Social organization of an urban grants economy" - a variant on Veblen's theme of conspicuous consumption. For the rich philanthropy is a form of conspicuous public consumption of wealth that manifests their high social status among peers - the more one wastes i.e. donates the higher the status. Needless to say that all that giving goes to conventionally "worthy" causes such as helping poor children or funding breast cancer research, to conceal the wasteful nature of it (per Veblen's analysis.)
For the working or lower middle class fractions - it is a form of social solidarity expression often defined by their religious affiliation i.. churches that require tithing and volunteering - hence the correlation between conservatism and charitable behavior.
As far as liberals and lefties are concerned, these tend to belong to or come from the upper middle class fractions (professionals, academics, artists, etc.) whose social status is built on individual meritocratic contributions (work) rather than conspicuous waste of wealth (philanthropy) or affinity to social group (e.g. church membership). Consequently giving away money does not enhance their social standing based on their class fraction membership. Hence they give less and thus appear "stingy." Volunteering otoh is more consistent with the work-based meritocracy, and that explains why college graduates volunteer considerably more than those without college degree http://www.bls.gov/news.release/volun.nr0.htm .
I think it can also help explaining the high levels of vitriol expressed by many lefties. Vitriolic debates and conspicuous consumption of esoteric works of art & literature are manifestations of the social status based on work- and mental prowess- based meritocracy, and thus using them liberally (no pun intended ;) aims at enhancing "cultural capital' (as Bourdieu would say) of this class fraction.
Wojtek