The thinking is that the existence of extensive gov't social welfare programs make people feel as if they have contributed (via taxes) and this reduce their personal responsibility to work on behalf of the less fortunate. Likewise, it undermines their empathy for the less fortunate because "well, they already receive welfare - so if they remain poor it must be because they [are lazy/addicted to drugs/antisocial/etc.]".
I don't know if I agree with this argument; however, the cultural ramifications of BIG are certainly an important question to wrestle with.
Edit: clarification - the argument is that this belief ("I've done my part") is extended because BIG covers all sources of welfare rather than targeted toward specific populations or purposes (children, mothers, food, etc). When everyone receives social welfare payments (rather than just specific kinds of people) it more fully extends the "I've done my part" feeling to the larger population (rather than specific slices of the population) and reduces overall charitable action.