Forced to pay tip for takeout at airport

I was at the departure lounge at Owen Roberts Airport and stopped and picked out a sandwich and a drink from the cooler. When paying for the items I was told there was an automatic 25% added. When I asked for it to be removed they refused as it was already built into the sandwich price. I explained to them that I was not served. I retrieved the items myself and brought them to the register to tab out. They still refused. My question is can they make you pay gratuity?

Read Auntie’s answer and comment on CNS Local Life