Do Martian bacteria (if they exist) have a "right" to Mars?

In my view, no bacteria have any right to anything. Any right is the right to exercise some option, so its prerequisite is some level of awareness and ability to make decisions. I don't think insects and plants are capable of having rights, nor rivers and forests as such.

That doesn't mean we should let people destroy them. That could be a big loss to the world.

For Martian bacteria, their extinction could destroy our chance to learn whether they are related to Earthly life, and that would be a great loss to our understanding of life in the universe.