I'm not sure that's the case at all. And even if they did assume that they got the full value, they would be quickly corrected of that belief when they actually filed their taxes.
Having volunteered to help people fill out their taxes, I think you vastly overestimate people's abilities to understand complex math. Most people just don't understand how deductions or graduated tax brackets work, even after having it explained to them.
Also, it's hidden. If you used TurboTax for example, you put all your numbers in and get a number out. All you've heard is "donations are deductible" but there really is no obvious connection to putting in your donations and seeing how much it saves you.
