Three explanations are possible:
- Biden is cheap and does not want to give money
- Biden gives cash to his local catholic church
- Biden thinks it is not right to claim tax deductions for charity, because that is just giving someone else's money away.
That is, compared to not giving, the taxman pays 25 per cent of your donation. Perhaps Biden thinks, liberal as he is, that the state has good use for this money and does not think other people should subsidize his charities.
UPDATE: Judging from the links to Mankiw's blog, concervatives have feasted upon this and not a single one has even considered this aspect. This blatant hypocrisy, double standards, etc. of the conservatives doesn't seem to bother anyone. And I am not surprised.