First Lunar habitats are going to be mostly underground anyway, and nuclear-powered. Not much is needed to terraform.
Digging underground is a reasonable way to get a massive layer of radiation protection without carrying it with you. This is important when you are just starting a long-term habitat. Later designs can of course be different.
Digging underground is reasonable if your only requirement is "block radiation". Its less reasonable when requirements also include 'get the digging machines into space', 'have it survive landing', and 'learn all the new fun techniques required to dig into lunar regolith in a low g environment with never before tested or used techniques and equipment'
Also since some rather missinformed protests back in IIRC cassini times I don't thin anyone really cares about modern RTGs being launched these days.
I think their ideal is to protect planets from interfering with their ecosystems unintentionally. Microbial contamination, for example. Especially Mars, since we don't know what kind of life, if any, existed or currently exists there. Also some of Saturn's and maybe Jupiter's moons. But I think if there were ever a strategic reason and viable option to terraform one of those bodies, which I think is pretty unlikely anyway, NASA would probably consider it. But by that time, I think it's extremely likely that NASA and the USA probably wouldn't exist as we know it anyway.
It does seem that unintentional interference is the main concern, but intentional interference almost certainly won't be considered on the relevant timescale, which is now until the first Mars landing, since aggressive terraforming was posed as an alternative to unsustainable Martian colonies.
[0] https://sma.nasa.gov/sma-disciplines/planetary-protection