I agree with you in part, but where does it stop? Do we stop motorcyclists wearing helmets, people dressed up wearing masks, mascots wearing teddy bear heads etc...
I understand where you're coming from, but it's also a matter of using common sense. The law is the law, but as this is only an offence which attracts a penalty (seemingly) I'm sure those who are enforcing it will always be allowed to use their discretion in applying the law.
The BBC article states:
French law says nobody can wear in a public space clothing intended to conceal the face.
From that I take it that there must be a specific intention to conceal the face. Wearing a motorcycle helmet for instance is not an intention to conceal the face as such but a requirement to which public policy for the protection of individuals safety will prevail.
As for Halloween and the like, I'm not too sure. But again presumably there will be a matter of proportionality and common sense applied. For instance, I doubt a bunch of 6 year olds are going to be stopped by the police on October 31st every year and given on the spot fines.
I think you're unnecessarily worrying about the intricacies of how it will work in practice.

Ha !