Its all about available space. More motorhome parking in France because it is a bigger country, but free motorhome parking in France is dying out, many seaside towns now have their 'Aire de campingcar' barrier controlled with creditcard entry charge as well coin in slot service facilities.
As pointed out many French tourist towns are clamping down on motorhome parking, mainly because it is getting out of hand, but there is a battle going on with the French campingcar club who obviously want free motorhome parking everywhere.
At one French seaside town there is a large unsurfaced area by the beach where motorhomes park, users have to pay & one wonders what owners of £60K motorhomes are getting out of parking in what resembles a refugee camp in Jul/Aug.
However, in quieter areas of France there is still no problem parking free overnight in town carparks etc, so its all down to using your sense & you can generally find free overnight parking even in the UK.