There does seem to be a reason behind this: preventing fraud:
"Mr. Six considers it in the interest of the breweries that publicans no longer sell Light Lager for Pilsener beer, since drinkers in some areas, such as the South, cannot tell the difference.
Messrs. Honig, Zylker and Smits van Waesberghe agree in principle.
De Heer Swinkels says that he only has Light Lager customers in Rotterdam. It will be very difficult for Spreker's brewery in connection with the heavy beer percentage to supply Pilsener beer to these clients, while Speaker wonders whether he will be able to get enough colouring for Dark Lager. The speaker is afraid of losing customers in Rotterdam.
Mr. van Waes agrees.
Mr. Smits van Waesberghe understands the objections of Heeren Swinkels and van Waes. The speaker wondered whether it is not possible to introduce the Kundenschutz already now with regard to the Light Lager issue among the breweries present here.
Minutes of the management of the CBK on 26th March 1941, held at the Amsterdam City Archives, document number 31121-1, page 211.
Of course. Publicans were passing off the cheaper beer as Pilsner. Now there's a surprise.
Why were Southerners singled out as having no discernment?
Can't sell Light Lager any more? Simple. Add caramel to it and hey presto: Dark Lager.
Sounds like all the brewers had agreed not sell Light Lager in Rotterdam. But they hadn't. Find out who the renegades were next time.