Eurasian Eagle Owls always look so grumpy

They're called eagle owls for reason.

I understand your concerns, and in the case of things like owl cafes or "pet" raptors, I totally agree.

This was at a falconry center, and while I don't know Dutch law, I can only assume it's equal or better than US law, which is very strict on how these birds must be cared for.

With licensed handlers working with these birds, if you think of the straps as seat belts, that would probably give you a better idea how they're used.

They're not tied when in their enclosure. They're not so much restrained when on the glove, as it is to keep them from getting spooked or into a situation they could be hurt.

There are also longer leads if they do need to be put on a perch in the open, but again, it doesn't so much restrict them moving around and being comfortable, it's to keep them from getting into hazards or hunting things that might have been exposed to poison and such.

So the majority of the time, they're not restricted, other than when they are on glove to move them or to educate people on birds. These are all falconry birds, and letting them go off and fly is the whole point of keeping them.