Therefore, they are not representative of the population at large, but are a technically biased sample.
but Chronicle editors suggest that a worrying trend has emerged: less philanthropic spending among Americans.
"Chronicle editors suggest..."
Identifying trends based on data known to be biased is, we may say, unhelpful. It is possible that Americans in general are less generous than in the past, but this data doesn't contribute much information.
If people pay less in taxes, then they have more disposable income. More disposable income could lead to more charitable giving, regardless of whether someone gets a tax benefit.
I give to a few charities. I take the tax deduction as allowed by law. But I would give anyway even if I didn’t itemize and get a tax deduction.
Do big numbers of people who give to charity do so only because of the tax deduction?