Not all chemists and dentists are good at checking that someone has a right to free prescriptions or free treatment.
I started taking metformin for reactive hypoglycaemia five years before I became diabetic and often had to remind my chemist that I had to pay for my prescriptions. They saw metformin on the list and automatically assumed that I was diabetic. Had I accepted my prescriptions for free I could have been fined for it.
Dentists tend to do the same, if you say you have an exemption they don't always ask for proof or even question what your exemption covers. They aren't losing out, they get paid by the government and if you've made a false claim to get free treatment you will be prosecuted for it, not them even though they should make sure you have the right to free treatment before they give it.