As above, guessing its for insurance.
Thats the rough jist of it.If you buy a uk car it has to be 'imported' from france and so if you have the alarm option specified they fit it as it comes off the boat in the 'import centre', they don't fit them at the factory.
If your car is an import, it didn't come into the country like a uk car would and hence misses the dock side fitting.
I think, someone may be able too confirm.