[rdlw]

Viability due to public opinion is a reason. I think that NASA has been successful enough in popularizing the ideals of not interfering with the potential biospheres (or lacks of biosphere) of other planets, 'Planetary Protection', that any plan for 'aggressive remote terraforming' would be met with public outcry, for the sake of preserving areological history.

[LargeWu]

"the ideals of not interfering with the potential biospheres (or lacks of biosphere) of other planets"

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.

[rdlw]

NASA states that the first goal of Planetary Protection is to "Carefully control forward contamination of other worlds by terrestrial organisms and organic materials carried by spacecraft in order to guarantee the integrity of the search and study of extraterrestrial life, if it exists." [0]

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