Ah, guess who makes the rules about how close you may be to an active volcano? It's usually the corresponding observatory, and they are valid for the public, not for the scientists.

And then, safe just means nobody gets hurt. The volcano will eat some instrumentation anyway at some point, probably. They often do. Plus, the sulfuric acid plume emissions will damage your gear if you get caught in it.

Something like a DJI Mavic Air (which is more like $1.5k than $1k) will happily get 5-10km range.

The risks associated with being 5km away piloting a drone and in a helicopter full of people overhead is radically different.

I mean, I wouldn't have wanted to be only 5km away from Mt St Helens when It blew its top ~40 years ago, but that one had spent 6 weeks giving off 4 and 5 magnitude earthquake warnings...

I think there's be a huge range of volcanos where it'd be risky to overfly in a helicopter risking human life, but where it'd be perfectly safe and sensible to fly drones for fun/curiosity/commercial-stock-footage/education/some-possible-eventual-science-benefit. I mean the major reason _not_ to overfly sites with drones are entirely absent, there's nobody and nothing valuable to hit and nothing for a drone battery to set fire to if you crash one on a lava field...