In europe at least people can choose about them own "image" being public or not, replication and use.
So first the law tell us that you should ask for it if you want to publish footage of people practicing.
After that you never know what are doing people if you don't ask them. So you don't know if they work on something or not. Maybe they think that convention or gym is a intime place where they can freely practice whithout anyone stealing, judging, like you know what happen in convention stay in convention state of mind.
If you have the urge to share something it's probably mean that is pretty new or neat or i don't know.
Maybe you share a random people doing a 3balls cascade but i think you share cool unknow manipulation, sequence of movement. So maybe it's exactly why people don't want it to be share publicly.
Maybe the aswered can change depend of whom asking for it. If people know that someone is sharing without any consideration, don't give any credit, share to all the juggling community or i don't know maybe they will act differently with him than a random personn who want to share something with them family.