[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