Donations made by customers at checkout are not tax-deductible for the business, as the donation does not come from the company. According to TPC, the business only serves as a collector for charitable donations from its customers and has no right to claim any of the collected funds.
If you got this far in the comment, take a mental note to call this out the next time you see it. I too am very critical of the late stage capitalist hellscape we live in, but rounding up for charity is a rare instance of an unproblematic practice that is damaging to discourage as this post does. Charities do a lot of good and when you donate to them you are the one that gets the opportunity to do a writeoff.
Edit: If you are wondering why they do it then, it’s a psychological marketing technique. If you come to associate the good things that the Ronald McDonald House does with your McBurger, you are more likely to buy more tasty McBurgers. Sketchy? Sure, but it happens to be really effective at supporting charity work so it’s kind of a mutually beneficial arrangement.