Why didn't they get the death penalty?

>If a firebender set a rock on fire and it melted, the Earthbender could still use it
And yet there are three times earthbenders had to contend with magma in TLA: none of them do so. Only Kyoshi and Roku (and also Roku while possessing Aang) were shown to render changes in lava while in the avatar state, and only once does it appear to be the intent of their bending and not a byproduct of other action. Not only is it clearly a retcon: Korra itself shows earthbenders lose the ability to control their rocks when they melt, so your point isn't even right in that context. Hell it kind of suggests lavabending is an opposed talent to metalbending which would only lead to speculating about what else is "technically earth but only for some people". But that's not here or there.

>In fact in the fricking show, he shows you how he lava bends by accelerating the rock as fast as possible till it catches fire and forms lava
Ghazan never does that when lavabending again. Bolin never needs to idle to charge up lava either. He clearly the three thumb-sized rocks together to make a rock boomerang then spun and heated it to make a superheated sawblade to cut through his wooden cage in quick order because Korra S3 was playing the hits of ATLA, and it took a good while for a handful of water to get Katara out of the same kind of cage.

>It's a big world
It REALLY is not. It can be circumnavigated in a few hours.

>It's a retcon in the same way that other people being able to learn
Except it's not that. Yes it's a new thing to show the audience. But it is not presented as an innovation or discovery in application of the same bending. It is presented as a talent. Bolin lucks into doing it when he wasn't trying to.
Toph didn't magically bend metal one day. When given no other option she used the special sense developed as a blind earthbender to find impurities in metal that she could bend like raw earth. And it took her hours at the very least to breech even the level to somewhat move it. No one else did it, because no one else combined all of those criteria.
Hama figured out how to sense and control water that's part of more complicated structures. Along the lines of what Huu did by bending plants. It took her years, and she was only able to employ it at the zenith of waterbending power.
Lightningbending appeared in the 7th episode with Firebenders so... this is meets the definition the least. It predates healing. But it is eventually explained that firebenders at root just direct energy and figured out polarized energy to make an electrical discharge like lightning. We aren't shown its origin but when Zuko can't lightningbend it isn't suggested to be because he's not special and chosen. It's not because he isn't good enough at firebending. It's because he can't complete the mental requirements.
No one questioned combustionbending because it was clearly just projecting energy in a similar but different way to both basic fire and lightning bending.

But again what the framing in Korra suggests is lavabending is a talent. Some earthbenders are just the right ones that get to do it. If we accept it was an innate talent it should have occurred before. The only justification for it only occuring after TLA is suggesting the talent is from crossbreeding firebenders and earthbenders.