Tipping Subway employees is complete BS anyway, please do not do it. They are not paid the waiter/waitress minimum wage, which has always been around $2-3/hour (it may be higher now). They are not servers and are paid much higher wages, even then some factory workers. Do not tip Subway, or Starbucks, anything like that. They are already compensated for their labor.
It could be the owners classified them as tipped employees if they brought food to patrons who ate in the restaurant. I am not sure what DOL considers the legit classification of servers who work for reduced minimum wage plus tips.