I used to think this too, and it's going around because it's excellent and plausible outrage bait.
...But as of now, that is not how it works. They are simply a collection point for the nonprofit / charity / whatever, and cannot use customers' donations to offset their own taxes.
I agree though, folks should just donate straight to good causes, but these "round up" campaigns actually generate a lot of money for charity organizations, and villainizing them is counter productive.
A few articles about this:
https://taxpolicycenter.org/taxvox/who-gets-tax-benefit-those-checkout-donations-0
All that said: Yes, scumbag corpos should be strongarmed to pay their damn taxes, and those taxes should go to the public good instead of warships and corruption!