I can't speak for this being true in the aikido world, but in many other martial arts they may be out right banned, need prior permisson, or may be considered poor form. I've been to a number of seminars in japanese sword arts where they were discouraged (by the Japanese guest instructors), and other arts where they were banned. They were banned for two seminars I hosted as well.
Seminars run by americans had a lot more liberal take on the presence of cameras.
In the YouTube era, I think most internet-savvy teachers are concerned about how out-of-context videos can be misinterpreted. I know Ikeda Sensei, for example, refuses to sell some of his videos to anyone except seminar attendees. He's afraid that they will look fake to anyone who wasn't actually present. It's a legitimate fear: there are plenty of examples of exactly that around these boards.