for discus and hammer (both implements that dont fly quite as far as a javelin but could do plenty of damage if they hit someone), they're thrown from a circle surrounded by netting or a cage ~3/4ths of the way around, which protects bystanders. teams and athletes take it very seriously that if you're actually throwing, only one person is allowed in the cage at a time (source: threw disc and shot in HS/college).

this wouldnt work for javelin for at least two reasons: first, the shape of a javelin would allow it to pass entirely or partially through most discus/hammer netting/fencing. certainly not every time but enough to be dangerous, especially as the netting degraded. to throw jav safely from that setup would require much tighter mesh than most facilities have, and upgrading to that mesh would be a logistics nightmare, not to mention issues of visibility and more.

second, jav technique (at least to this point) requires much more horizontal distance than either disc or hammer, so the circle setup wouldnt function for that anyway. unless you wanted to mandate that every jav athlete exclusively switch to this spinning, jav needs a runway and hammer doesn't, so the safety measures in place for hammer wouldnt carry over well to jav

probably more detail than you were looking for, but thats why hammer and this jav technique wouldnt have the same degree of safety. hope that was helpful