Symbolism, Imagery, Allegory
It seems that, whenever something important happens in Little Brother, coffee and/or alcohol aren't far behind. (When something important happens in our lives, there's usually a half-blue raspberry, half-cherry Slushy involved. But it takes all kinds.)
Marcus's parents won't let him drink coffee at home—maybe because his mom's a tea fanatic from Blighty—but he buys it on his way to school anyway. That Marcus: such a rebel.
But he's not just doing it for the love of that sweet, sweet bean juice. Coffee has some symbolic resonance for young Yallow.
And Marcus' interaction with the Turkish owner at his fave coffee shop, who says "my shop will not help [the government] spy on my customers" (6.10-12) through credit card transactions becomes a way for Marcus to understand freedom and government surveillance. Coffee also gives Marcus's friends a reason to meet up (as well as fueling late night coding sessions).
More than that, though, coffee symbolizes adulthood for Marcus. And he does tend to get his java on when he's dealing with extra-adult situations. When he and Ange meet with Barbara at the office to tell more about the Xnet, they all have coffee together. And he ends up drinking coffee with his parents, Ron, and Barbara at Barbara's house.
But Marcus does respect the legal drinking age…at least around the over-twenty-one set. Although whiskey is poured at Barbara's house, Barbara pours out "[…] four small cups. None for [Marcus]" (16.92-93).
At least according to the adults, Marcus isn't a member of the adult beverage club yet. (Maybe this is why Marcus throws a "Don't Trust Anyone Over 25"-themed party?)