Profanity on clothes?

Here's the issue my 16 year old step-grandson and my 34 year old step-daughter live with us. "Sam" the grandson has lived with us for 4 or 5 years now. His mother Lilly has been here 1 year, this year for school clothes she let him pick out several tee shirts that have the "F" word on them.He cannot wear them to school so he wears them here. Yesterday I saw the first one needless to say I put my foot down about it. Bottom line I said it was disrespectful and I do not want to see them again. Well now they are mad at me, saying he hears it everyday. Not from me! but his Mom says it as a matter of fact word. I replied so that makes it right? What do you think? Stick to my beliefs or am I wrong?