I had never heard about it being a tax writeoff, which wouldn't have made sense to me anyway, but I have heard many people question why multi-million dollar companies subsidize their donations through customers rather than just donating or matching customer donations.

They can and sometimes do. The checkout line donations also serve as a kind of marketing for the company. Charities also really like check out donations because they’re one of the more effective ways to solicit donations.

Donate a dollar at the register? Checkout charity is big business for nonprofits

It happens at stores everywhere. A cashier asks you to donate a dollar or two to charity while you're standing at the cash register.