Can I legally charge tip to anyone in my restaurant?

I have a $3.99 special that includes an enchilada with beans and rice and a fountain drink. These old couple come in and get that special. I always make sure they're doing okay and I believe I've given them excellent service. The problem is that they never leave a tip! Their bill with tax comes out to 8.60... show more I have a $3.99 special that includes an enchilada with beans and rice and a fountain drink. These old couple come in and get that special. I always make sure they're doing okay and I believe I've given them excellent service. The problem is that they never leave a tip! Their bill with tax comes out to 8.60 and they never leave one penny.
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