Suicide over Paracetamol Sales in the UK... Ignorance of UK Law and Regulation

Now, this is probably not going to be a great big heated discussion... but something happened to me yesterday that really shocked me! 

I was in a supermarket (ASDA, Sainsburys, Tesco) one of them... and I got refused to serve for a pack of paracetamol and ibuprofen. Now I actually made the manager of the store speechless as I questioned; why it was that it's illegal for them to sell me 20 of the most harmless tablets in existence, when I could very legally buy 400 bottles of vodka and 400 packets of tobacco? 

Speechless manager aside.. I handed my 2nd packet to my friend who bought them for me. 

WHY is it.. that the law can be so unbelievably ignorant, blind sighted and down right stupid? Is there anything we, as "normal" people can do to just say... "pfft! Get lost, that's just stupid"