Pourquoi dit-on en Belgique : 'Je vais à la toilette' et en France : 'Je vais aux toilettes'?

Parce qu’en France il faut en visiter plusieurs pour en trouver une qui est propre.

A French joke:

Why do the Belgians say, 'I need to go to the toilet (singular)', and the French say, 'I need to go to the toilets (plural)'?

Because the French have to go to several before they can find one that is clean!

Haha. This is a question I posed my French fiancé on the confusing grammar rule when it comes to always using the plural toilets even though there is clearly only one toilet, at home, for example. His very convincing reason, 'Just because,' didn't cut it out for me, but now I understand better! ;)