Don't worry: It's not illegal.

BEVERAGES
Many people overdo it with the drinking on St. Patrick's Day, but it's not always Guinness or Jameson that gets them into trouble. Sometimes it's the Shamrock Shake, McDonald's uniquely green and often elusive seasonal treat.
From Coca-Cola to carbonated water, there isn't a fizzy drink around that tastes better once it's gone flat.
These are the sweetest, messiest, most decadent hot chocolates around.
Maybe your barista isn’t so careless after all.