It is line of sight related.
Model planes below 400’ and within line of sight of the operator are okay.
It is operation of RPVs that are outside line-of-sight (and in weather) that presents the problem.
How does a RPV beyond line of sight ‘see and avoid’ another platform/aircraft searching as well?
Sense and avoid technology is emerging and has a long way to go.
Searching using RPVs can be done beyond line of sight and airspace notices (NOTAMS) have to be issued because many other manned/unmanned platforms will also be in the area, either as part of the search or flying generally.
If the RPV hit a manned aircraft and it crashed then the coverage would be hostile to letting RPVs fly into such a dense search area.
Everyone knows that having a human pilot on board makes all aircraft safe!