Don't feel ashamed, this level of cheapness has nothing to do with your nationality.
My dad was a physician, born and raised in the US. He earned a good living, worked very hard (he never had fewer than 2 jobs the whole 3 decades I knew him), and saved money religiously. And he was very frugal as well: he never went into debt for anything, drove cars till well after they fell apart, had clothing that I know was older than I which he wore till the day he died.
One of his great loves, though, was to steal things from the hospitals he worked at. Anything that wasn't bolted down - office supplies, filing cabinets, tables, chairs, desk lamps. At one point, he was bringing home hard boiled eggs that he pocketed in the staff room, because obviously we couldn't afford food. Several times when we were doing remodeling at home, I got to keep a lookout for security while we snuck bags of construction waste into the dumpsters at work.
Now, you didn't know the guy so it might not be obvious, but this was all about not spending a cent more than he had to. So, don't feel ashamed because some of your fellow country men are taking soda from work.
And I don't think what you said was prejudiced either.