Think I'm getting old...!

I'm confused (again!!)  It's a long time (over 30 years) since I studied law, and I know a lot has changed in the interim, but I always thought - and was taught - that the common law was indestructible.  Statute law could over-ride it but it remained there and if the statute was ever repealed it would spring back into life. Furthermore, if Parliament wished to pass an Act which over-rode common law, they needed to specifically spell that out.  Why am I going on like this? Well a bakery in Northern Ireland has been found guilty of discrimination for refusing to bake a cake for a customer which carried a message with which they disagreed.  Now my understanding of the common law position is that no shop is under a legal obligation to sell anything to anybody (basic contract law) so their reasons were immaterial - they had the right to refuse the order (or for that matter any order). So how can you reconcile that with the court's decision?  Like I say - confused.