This seems odd.
I am French and switch operators every year (because they have those stupid offers with significantly cheaper subscription that only last 12 months and it only takes a couple of minutes to subscribe to a new one) and I have used tethering frequently with Free, Numericable, Bouygues and SFR without getting any warning or noticing any oddity in the content served.
Also, Orange has a bit more inertia than the others (it used to be the French public phone company France Telecom), but their support page says that tethering is part of the regular mobile subscriptions.