You are confusing two kinds of lines.

Most American grocery store lines are due to restricted "checkout bandwidth." The Soviet grocery store lines were due to limited availability of goods. In the US you place the goods in your cart and wait to check out. In the USSR (at the time) you waited in line to get the goods themselves.

Sometimes you see the other kind of line in the US as well: for example, people stand for hours in long lines in the wee hours of Black Friday to buy a discounted TV. This is caused by the scarcity of these TVs. In the USSR the problem was the scarcity ("deficit") of essential goods (food, soap, toilet paper), and people had to stand in long lines to get them.