Earlier quoted context omitted.
So Earth bacteria will go out and contaminate Mars. sooner or later it would happen. We could still study the original Martian biome, if that exists.
The problem is, with Earth life present on Mars you can no longer be sure that the life you find there wasn't brought in by colonists.
We would be able to identify an extraterrestrial cell as surely as if it was carrying flashing lights and screaming "I'm an alien". While all life is based on information, and information needs to be somehow encoded in a, let's call it, genetic code, would that code be DNA and RNA based? would in encode proteins, and if so, would the building blocks be the same 20 amino acids that we have an Earth. Would the genes be encoded using the same nucleotites, and would "letter" in the genetic code be encoded by 3 nucleotides? If so, would always the same 3 nucleotides encode the same aminoacid as on Earth? etc, etc.
There are so many places that life could be different, that the chance of mistaking Martian life for terrestrial life is vanishingly small.