I've just been in a shop in Amsterdam which had a sign saying:
No pictures allowed and specially not when wearing our garments!!!
If we see you making pictures anyway, you will have to buy them by law!!
To my British sensibilities, it seems highly unlikely such a law would actually exist anywhere in Europe, as despite what cultural variation there is, this seems a bit extreme—it's one thing to require you to buy something you break, as is a done thing in some places, but it's a whole different class when you've not deprived the store of anything of value.
Can they oblige you to buy something in such a way? If you they try to, what should you do?