Something that harms a cashier (a worker) is harmful to a worker, which is for most intents and purposes "human."
"Robots are stealing our Goddamned jobs!" is a popular sentiment around the globe. Who is using these robots to replace jobs that should belong to people? Corporations. Corporations are not people, as much as some sorts seem to believe they are.
Taking advantage of a corporation's mistake hurts billionaires and millionaires (this includes McDonald's franchisees, as the corporation doesn't allow just anyone to become one, costs are high), who aren't really people in the conventional sense of the word.
These corporations, billionaires, millionaires and so forth are the ones who are stealing from the people; taking advantage of their errors for something like this is good. McDonald's spends millions a year on lobbying to harm the common good; $5 or $10 or even $1,000,000 from them will mean nothing to them, but will go a long way for the people taking it.
Taking advantage of McDonald's Corporation errors is morally no different than shoplifting from Wal*Mart, which similarly spends millions lobbying and harming the common good, the only difference is that the former presents no risk to the person doing it.
In an era where "No one should be a billionaire!" is a popular political viewpoint, and billionaires have effectively stolen the common person's political agency through lobbying, it seems reasonable that people won't see any harm in taking stuff from them, especially small amounts that they won't miss, like this.
I don't necessarily agree with the sentiment, but it's definitely reasonable and understandable to see why people don't care about taking from people who, from their perspective, didn't earn it in the first place, and won't notice that it's gone.