Why don't the stores ever give customers the option of rounding down for charity at checkout?

Because you can't vote for them. This only happens in vote buying.

They don't care for you being a charity.
They're collecting for other subscribed charities and that only happens with rounding up.

Round of error would cost them thousands if not far more. Banks learned that lesson back in the 1960s where someone collected millions from the round off error.

the stores where i am at often do