I read the thread to see what this informative explanation was.
It was bollocks. Someone said that food packaging rules say that you have to state where ingredients are from. There are rules for fresh meat origins and geographical indicators where there's a possibility of misleading (including protected designations). There is no rule to cover salt and pepper origins.
Morrissons clearly did that as a xenophobic badge of honour.
That said, the frothing mess of the comments on that thread is quite something. Lots of "i'll take my business elsewhere" twattery.
In summary, twitter is full of fannies.