see and avoid
In the manned airplane the responsibility is equally shared between both pilots who have to look, see and avoid.
When flying let's say a GA plane, and closing in on a drone, the situation is not equal.
The drone is small, has very little surface to see and is most of the times black, and is stationary or moves at a slow speed. The latter means that Your eyes don't remark a movement against the background, which is mostly also darkish.
When meeting a "classic" model airplane, it will be different as it will be moving in a way that it protrudes from the background, moreover, many have bright colours on a relative large area.
The drone pilot will hear a plane or helicopter approaching first. Then, while scanning the sky, he will see the airplane, which is clearly visible against the sky backdrop.
He will have time enough to descend or get out of the path of the plane.
So there lies a big responsibility in the hands of the drone operator and they should be made aware of this. Many "ground pilots" have no idea how the view of the pilot in the plane might look like and that his drone will only be seen in the last moment at close proximity.
Transponders: apart from the weight and cost, ask big airport controllers what they would think of a tsunami of "blips" cluttering their screens even more. Other planes would not benefit directly, only a few GA planes have a TCAS system, which is VERY expensive.
If drones are used in a responsible way, there are many benefits possible, we are just at the beginning of what will eventually become a new aviation branch, creating many jobs.
Please do not only stare at a few over exposed (worldwide)incidents. The drone risk is minimal compared to the bird strike risk.
However, both hobbyists and commercial operators will have to be made aware of they risk they can be for manned aircraft. Most of the "classic" model airplane pilots already know, but now the new group needs to be made aware. If information does not work, publicising expensive penalties or jail time for irresponsible drone pilots will surely help.
It worked getting passengers think twice before starting an "air rage".