Not only do most people find it difficult to name ten famous Belgians without falling back on Tintin and Hercule Poirot, but Belgium seems to be the only country name that English-speaking people get mixed up with its adjective. From The Independent on Saturday 21 November 2009:
Sweet treats are available at Marks & Spencer this week with half-price Belgium chocolate selections. Both a 480g Belgium chocolate selection and a 1kg tin of Belgium chocolate biscuits are reduced from £12 to just £5.99.
So, not simply a slip of the fingers in typing, but a belief by the writer that this was the correct form. The BBC refers to its “Scotland correspondent” and its “Rome correspondent”, and it would be appropriate to talk of a “Belgium correspondent”. Using the adjective in these cases would imply that the correspondent was Scottish (in the case of the Scotland correspondent, he/she usually is), Italian (from Rome) or Belgian respectively, as opposed to a correspondent writing about that country. However, there is nothing similar in the use of the country name when referring to chocolate, so the adjective is the one to use.