Apparently it is illegal in Portugal, Austria and Luxembourg to have dash cams in your car and will result in large fines if caught, even if they are off or stored anywhere in your vehicle such as the glove box. Does this then also mean helmet cams will fall into the same category?
Not quite that clearcut they are illegal in Portugal. Reading this article they can still be used, it is how the footage is used may cause some legal rangling. I noticed that when I searched for the same for Austria, all the "newspaper sites" are saying illegal and fines up to £"stupid money", but when you dig deeper it is the same as Portugal. It is not the use of the camera but how you use the footage. See here.
In most of the U.S., the police love them as they are n impartial witness. Insurance companies love them as it can show their insured was not at fault. I've seen people trying to commit insurance fraud be stopped when the target points to the camera in the windshield. Now, one place I DO cover or remove the dash cam is when I go to a military base. Photography of those is not allowed here and I'm sure most other countries are the same way.