If there's an item of American culture you may be missing, it's probably that American restaurants can be very cheap.
This arose because the easiest way for a restaurant to increase revenue is to increase portion sizes and hence the price.
"What? Was there something wrong?"
"No, i'm in a hotel and don't really have anywhere to store or reheat it."
"Yeah, but... Are you sure you want it?"
Of course, high-end restaurant also do exist.
The english language varies considerably by geographical region, the speaker's socioeconomic class, etc. Your statement is not universally true; I've met people to whom it's true and people to whom it's not.
The original comment said that Americans consider McDonald's a restaurant, in implicit contrast to unnamed other places where McDonald's does not qualify as a "restaurant," as a way to suggest that Americans have lower standards than others for food quality.
I am pointing out that, at least among the people I've known my whole life here in the US, this is definitely not the case; McDonald's and other fast food places are not in any sense considered "restaurants," no more than a gas station that sells microwave burritos is a "restaurant."
I hope you limbered up sufficiently before stretching so far to find a way to take that as an insult.
A restaurant would be anywhere that a group can go to have food prepared for them, and then sit down and eat it. Most gas stations don't qualify on either count.
(And incidentally I've been to some really excellent, chef-owned restaurants that don't have table service. Cajun places, mainly. Cajun seems to occupy a weird intersection between "classy" and "cheap" that no other cuisine can enter.)
Key point: apparently McD is cheap in the United States. Not so much in the eastern parts of Europe.