One gets accustomed to the differences between the French and British tax and legal systems, but sometimes the consequences of those differences can lead to outcomes that seem bizarre.
In the UK, you carry a personal responsibility to declare your income, and you are liable for tax and social security on it. It is generally down to you, the individual, to be honest. In France, the employer carries the responsibility, and is required to declare and make payments on your behalf. These payments can be onerous, sometimes greater even than the base salary, and the crime of employée dissimulée (hidden/clandestine employment) is on the books, with serious penalties, to dissuade people from dodging the system.
The term "employment" covers everything that you would expect, including what we would describe as self-employment, and for anything. So if you're in France and your mower packs up and you pay me 20 quid for me to cut your lawn with my mower, you're breaking the law. (In the UK, the law would only be broken if I failed to declare the sum earned at the end of the tax year.) It covers payment in kind too: if instead of paying me twenty quid, you invite me to dinner, you're still guilty. Taken to its logical conclusion, this law prohibits the normal intercourse between friends and neighbours.
This can have strange consequences for everyday life. For example, I am a member of the Harmonie (wind band) at Sainte Suzanne, and the annual membership subscription is a token euro. However, wives, husbands or partners of members are often invited to join in the functions, dinners, visits, etc. So we pay 2 euros per year subscription so that if Anita helps out at any event, and gets a free drink or meal in return, the Harmonie can't be accused of the dreaded employée dissimulée, because she is a member too.
As another example, there was an altercation that hit the press a while back, between the powers that be and a Muslim woman. An investigation of her circumstances led to the discovery that her husband, although legally married only to the lady in question, had several other "wives" according to his religion. One of the crimes he was then accused of was employée dissumulée, and I always wondered if this related to financial payments he made to his other partners, and whether the accusation would be of prostitution, house-cleaning, or perhaps restaurant services. The implications for anyone having an extra-marital affair, which has been known to include French presidents, could be disastrous.
I was surprised to read an article in a paper today. If I understand it correctly, a bar owner is being sued for employée dissimulée because it is the custom, at certain of his organised evening events, for clients to carry trays of used glasses back to the bar, thus being "employed" as waiters. The criminal complaint having been found to have no merit, the suing party is now going for civil damages of several thousand euros.
Quite bemusing for those of us brought up to understand the word liberté as meaning something rather more... well, free. I am also guessing that since tax and social security revenues are generally proportional to employment levels, someone, somewhere, is hurting for money.