I mentioned this story to my family and my daughter said that there are starbucks in brooklyn that are kosher (can even get CY milk upon request), I don’t know this for a fact but perhaps these women think the starbucks here is kosher as well. Then it would be a mitzvah to tell these ladies that the Starbucks has no hechsher (of course in a nice way)and if they continue eating you have done your job.
Just to add on, if you are at the mall and want to get Haagen Daas icecream from the vendors, or any other snack, it is important to look at their certificate of kashrus to see what exactly is kosher and what is not.